To use Team O'clock behind a firewall you need to whitelist the service's IP addresses. However, Team O'clock is using dynamic ip addresses, hosted in the us-east-1 AWS region.
Although not recommended, you can still whitelist the entire AWS US East Region using the following IP prefixes from the AWS IP Ranges documentation:
15.230.56.104/31 52.93.127.163/32 3.2.0.0/24 15.230.137.0/24 52.4.0.0/14 50.16.0.0/15 52.95.208.0/22 52.93.127.169/32 52.94.244.0/22 64.252.69.0/24 15.230.56.130/31 52.93.50.128/32 52.93.50.140/31 52.95.41.0/24 176.32.125.244/31 3.4.0.0/24 52.93.91.102/32 13.34.29.224/27 13.248.124.0/24 18.232.0.0/14 150.222.2.0/24 52.93.50.136/31 52.93.50.166/31 54.156.0.0/14 54.236.0.0/15 99.150.8.0/21 15.230.130.0/24 52.93.50.146/31 52.93.50.156/31 52.93.127.124/32 52.144.193.128/26 107.20.0.0/14 15.230.40.0/24 52.93.50.174/31 15.230.56.90/31 52.93.51.28/32 52.93.91.101/32 15.230.66.0/26 52.46.252.0/22 52.93.50.142/31 150.222.227.0/24 13.34.29.128/27 52.119.206.0/23 176.32.125.230/31 52.93.127.218/32 205.251.244.0/23 52.93.50.178/31 52.93.50.188/31 52.93.236.0/24 54.239.98.0/24 176.32.125.228/31 13.34.30.128/27 54.144.0.0/14 15.230.56.190/31 52.93.127.69/32 75.101.128.0/17 176.32.125.234/31 52.144.192.0/26 13.34.30.160/27 67.202.0.0/18 54.226.0.0/15 162.250.237.0/24 15.230.56.94/31 64.187.128.0/20 99.82.188.0/22 15.230.18.0/24 52.93.127.201/32 150.222.79.0/24 3.224.0.0/12 15.230.56.92/31 52.93.50.162/31 52.93.50.184/31 52.144.192.192/26 54.221.0.0/16 54.240.202.0/24 15.230.56.134/31 52.95.63.0/24 52.93.126.215/32 52.93.127.217/32 68.66.112.0/20 54.239.112.0/24 34.192.0.0/12 52.95.216.0/22 52.119.232.0/21 54.231.244.0/22 150.222.99.0/24 150.222.218.0/24 176.32.125.250/31 52.93.127.167/32 54.239.108.0/22 64.252.68.0/24 52.93.50.148/31 52.94.68.0/24 99.77.191.0/24 15.230.56.102/31 15.230.56.152/31 44.192.0.0/11 15.230.56.156/31 52.93.50.129/32 54.239.8.0/21 207.171.176.0/20 162.250.238.0/23 150.222.100.0/24 15.230.56.168/31 52.93.50.130/32 13.34.30.96/27 52.93.127.219/32 13.34.29.96/27 52.93.127.18/32 176.32.125.252/31 15.230.56.140/31 205.251.246.0/24 15.230.56.120/31 23.20.0.0/14 52.46.168.0/23 52.93.91.115/32 64.252.64.0/24 3.4.1.0/24 72.21.192.0/19 15.230.56.124/31 15.230.56.172/31 54.231.0.0/17 52.93.126.123/32 52.94.124.0/22 52.93.50.176/31 54.240.216.0/22 99.82.166.0/24 52.93.91.96/32 52.93.91.105/32 52.44.0.0/15 13.34.30.64/27 15.230.56.184/31 52.93.91.111/32 52.93.123.255/32 15.230.56.106/31 15.230.56.186/31 205.251.247.0/24 15.193.6.0/24 52.93.127.182/32 52.90.0.0/15 52.93.91.114/32 13.34.30.0/27 52.0.0.0/15 15.230.56.170/31 52.54.0.0/15 52.93.3.0/24 52.93.50.182/31 52.119.196.0/22 52.93.91.100/32 150.222.71.0/24 52.93.50.190/31 13.34.29.160/27 15.230.56.86/31 99.77.129.0/24 99.82.171.0/24 15.230.56.72/31 52.144.200.64/26 13.34.31.64/27 15.230.56.108/31 35.153.0.0/16 52.144.192.128/26 52.216.0.0/15 162.250.236.0/24 52.46.250.0/23 150.222.205.0/24 15.230.56.98/31 15.230.56.178/31 52.93.91.112/32 52.200.0.0/13 15.230.56.128/31 34.224.0.0/12 13.34.31.96/27 15.230.56.114/31 52.93.127.221/32 52.144.195.0/26 52.93.50.164/31 52.93.64.0/24 13.34.29.64/27 15.230.56.154/31 50.19.0.0/16 150.222.73.0/24 15.221.4.0/23 15.230.56.110/31 52.93.127.172/32 3.2.3.0/24 15.230.38.0/24 15.230.56.76/31 52.93.127.68/32 174.129.0.0/16 52.95.255.80/28 150.222.87.0/24 15.230.65.192/26 18.208.0.0/13 52.95.245.0/24 99.77.187.0/24 176.32.125.254/31 52.93.123.98/32 176.32.125.236/31 52.93.91.108/32 52.93.127.122/32 99.82.167.0/24 52.93.50.180/31 184.72.128.0/17 205.251.248.0/24 15.230.56.182/31 15.230.56.66/31 15.230.56.126/31 54.80.0.0/13 15.230.142.0/24 52.20.0.0/14 52.94.201.0/26 15.230.56.150/31 54.242.0.0/15 216.182.238.0/23 99.83.101.0/24 150.222.224.0/24 13.34.31.32/27 52.94.192.0/22 15.230.56.78/31 52.95.52.0/22 52.93.254.0/24 99.82.175.0/24 99.83.88.0/21 52.119.214.0/23 208.86.88.0/23 15.230.56.142/31 54.208.0.0/15 99.83.64.0/21 15.221.24.0/21 150.222.237.0/24 15.230.56.100/31 52.93.127.180/32 15.230.56.64/31 15.230.56.84/31 52.93.50.144/31 52.94.224.0/20 99.77.128.0/24 15.230.56.82/31 150.222.226.0/24 54.152.0.0/16 176.32.125.240/31 54.240.208.0/22 52.70.0.0/15 52.94.248.0/28 52.119.212.0/23 52.93.50.168/31 52.93.127.19/32 52.93.127.216/32 99.77.254.0/24 13.248.103.0/24 15.230.56.176/31 176.32.125.232/31 3.5.0.0/18 15.177.64.0/23 52.86.0.0/15 52.93.91.113/32 52.93.97.0/24 15.230.56.252/31 52.93.127.162/32 205.251.224.0/22 15.230.56.112/31 54.172.0.0/15 52.93.127.200/32 18.204.0.0/14 52.93.91.110/32 54.88.0.0/14 99.78.192.0/22 52.95.62.0/24 15.230.56.136/31 52.93.50.134/31 52.93.127.183/32 3.2.2.0/24 52.93.50.131/32 64.252.66.0/24 150.222.110.0/24 3.3.5.0/24 52.2.0.0/15 52.93.127.125/32 176.32.96.0/21 184.72.64.0/18 13.248.108.0/24 52.95.108.0/23 52.144.193.0/26 52.93.123.136/32 52.95.48.0/22 54.240.232.0/22 150.222.143.0/24 52.119.224.0/21 216.182.232.0/22 15.230.56.138/31 15.230.56.118/31 150.222.223.0/24 15.230.65.128/26 15.230.145.0/24 52.93.50.138/31 52.93.50.152/31 199.127.232.0/22 99.82.176.0/21 204.236.192.0/18 35.168.0.0/13 72.44.32.0/19 52.93.51.29/32 52.93.1.0/24 52.93.50.160/31 52.93.50.132/31 150.222.236.0/24 3.4.2.0/24 13.34.31.0/27 52.46.170.0/23 54.92.128.0/17 54.239.0.0/28 99.77.151.0/24 216.182.224.0/21 52.93.91.98/32 54.204.0.0/15 15.230.56.148/31 54.196.0.0/15 176.32.125.226/31 52.94.252.0/23 15.230.35.0/24 52.93.249.0/24 207.171.160.0/20 15.230.56.166/31 13.34.30.32/27 15.230.56.132/31 52.72.0.0/15 52.93.126.212/32 176.32.125.192/27 176.32.125.238/31 15.230.56.146/31 54.160.0.0/13 15.230.56.254/31 52.93.127.164/32 54.234.0.0/15 52.92.16.0/20 172.96.97.0/24 52.93.126.122/32 52.94.254.0/23 52.93.127.185/32 15.230.56.164/31 3.208.0.0/12 15.221.0.0/24 15.230.56.70/31 52.93.91.103/32 176.32.124.128/25 184.73.0.0/16 54.174.0.0/15 52.93.4.0/24 52.93.59.0/24 54.224.0.0/15 99.82.165.0/24 52.93.50.194/31 3.80.0.0/12 15.230.56.68/31 15.230.56.174/31 52.93.50.154/31 52.93.126.213/32 52.144.200.128/26 54.240.196.0/24 150.222.66.0/24 52.93.127.165/32 13.34.29.192/27 176.32.125.246/31 15.230.56.116/31 150.222.222.0/24 52.93.127.181/32 52.93.126.214/32 52.93.127.173/32 64.252.67.0/24 54.198.0.0/16 13.34.30.192/27 15.230.56.250/31 52.46.128.0/19 52.93.127.166/32 176.32.125.242/31 15.230.56.88/31 52.93.91.109/32 52.93.127.123/32 52.93.50.186/31 52.144.193.64/26 54.239.16.0/20 13.34.30.224/27 15.230.56.180/31 52.144.194.0/26 52.46.164.0/23 15.230.56.188/31 52.93.91.106/32 54.240.228.0/23 176.32.120.0/22 15.230.56.96/31 15.230.56.158/31 52.93.50.170/31 52.93.50.192/31 72.41.0.0/20 54.210.0.0/15 150.222.76.0/24 13.248.116.0/24 15.230.56.74/31 52.93.76.0/24 52.93.91.99/32 52.93.127.184/32 100.24.0.0/13 15.230.56.248/31 150.222.136.0/24 52.93.127.220/32 52.93.91.97/32 52.93.91.107/32 52.93.127.168/32 54.239.104.0/23 15.230.56.122/31 15.230.56.144/31 150.222.206.0/24 209.54.176.0/21 15.230.56.162/31 15.230.57.0/24 52.93.50.172/31 52.94.240.0/22 150.222.138.0/24 15.230.56.80/31 52.46.166.0/23 52.94.0.0/22 205.251.240.0/22 52.144.192.64/26 15.230.56.160/31 176.32.125.248/31 52.93.60.0/24 52.93.50.150/31 52.93.91.104/32 52.93.123.99/32 176.32.125.224/31 52.93.50.158/31 99.77.254.0/24 107.23.255.0/26 54.243.31.192/26 54.231.0.0/17 52.216.0.0/15 3.5.0.0/18 52.92.16.0/20 3.218.180.0/22 52.119.232.0/21 52.119.224.0/21 52.94.0.0/22 3.2.0.0/24 52.4.0.0/14 50.16.0.0/15 64.252.69.0/24 3.4.0.0/24 18.232.0.0/14 54.156.0.0/14 54.236.0.0/15 99.150.8.0/21 107.20.0.0/14 54.144.0.0/14 75.101.128.0/17 67.202.0.0/18 54.226.0.0/15 162.250.237.0/24 64.187.128.0/20 3.224.0.0/12 54.221.0.0/16 68.66.112.0/20 34.192.0.0/12 64.252.68.0/24 99.77.191.0/24 44.192.0.0/11 162.250.238.0/23 23.20.0.0/14 64.252.64.0/24 3.4.1.0/24 52.44.0.0/15 15.193.6.0/24 52.90.0.0/15 52.0.0.0/15 52.54.0.0/15 99.77.129.0/24 35.153.0.0/16 162.250.236.0/24 52.200.0.0/13 34.224.0.0/12 50.19.0.0/16 3.2.3.0/24 174.129.0.0/16 52.95.255.80/28 18.208.0.0/13 52.95.245.0/24 99.77.187.0/24 184.72.128.0/17 54.80.0.0/13 52.20.0.0/14 52.94.201.0/26 54.242.0.0/15 216.182.238.0/23 208.86.88.0/23 54.208.0.0/15 99.77.128.0/24 54.152.0.0/16 52.70.0.0/15 52.94.248.0/28 99.77.254.0/24 3.5.0.0/18 15.177.64.0/23 52.86.0.0/15 54.172.0.0/15 18.204.0.0/14 54.88.0.0/14 3.2.2.0/24 64.252.66.0/24 3.3.5.0/24 52.2.0.0/15 184.72.64.0/18 216.182.232.0/22 204.236.192.0/18 35.168.0.0/13 72.44.32.0/19 3.4.2.0/24 54.92.128.0/17 99.77.151.0/24 216.182.224.0/21 54.204.0.0/15 54.196.0.0/15 52.72.0.0/15 54.160.0.0/13 54.234.0.0/15 3.208.0.0/12 184.73.0.0/16 54.174.0.0/15 54.224.0.0/15 3.80.0.0/12 64.252.67.0/24 54.198.0.0/16 72.41.0.0/20 54.210.0.0/15 100.24.0.0/13 13.248.124.0/24 99.82.166.0/24 99.82.171.0/24 99.82.167.0/24 99.83.101.0/24 99.82.175.0/24 13.248.103.0/24 13.248.108.0/24 99.82.165.0/24 13.248.116.0/24 15.177.64.0/23 18.206.107.24/29 18.233.213.128/25 3.216.135.0/24 3.216.136.0/21 3.216.144.0/23 3.216.148.0/22 3.217.228.0/22 3.218.180.0/25 3.218.180.128/25 3.218.181.0/25 3.218.181.128/25 3.218.182.0/25 3.218.182.128/25 3.218.183.0/25 3.218.183.128/25 3.227.250.128/25 3.231.2.0/25 3.234.232.224/27 3.234.248.192/26 3.235.112.0/21 3.235.189.100/30 3.235.189.96/30 3.235.202.128/26 3.235.26.0/23 3.235.32.0/21 3.236.169.0/25 3.236.169.192/26 3.236.32.0/22 3.236.48.0/23 3.236.94.128/25 3.237.107.0/25 3.238.167.0/24 3.238.212.0/22 3.239.232.0/24 3.83.168.0/22 3.91.171.128/25 34.195.252.0/24 34.226.14.0/24 34.228.4.208/28 35.172.155.192/27 35.172.155.96/27 52.23.61.0/24 52.23.62.0/24 52.55.191.224/27