Warning: This post is over 4 years old, so some of what you read might be out-of-date.
I did something a bit stupid yesterday... I was playing around with Firewall rules on my Forge server. I setup a rule so that the only place I can SSH into my server was from my home IP Address.
However, as soon as I did that, I realised that I could not longer deploy code or do anything via Forge on that server. Then I realised my mistake.... I blocked Forge's IP Address from SSH'ing to my server.
In case someone else finds them in my position, here are the two commands I ran to fix the issue. Of course, you need to SSH to run them.
sudo ufw allow from 159.203.161.246 to any port 22
sudo ufw allow from 159.203.163.240 to any port 22
Forge uses ufw under the hood to manage firewall rules so that's what I used to manually give Forge back control. The two IP addresses I listed in those commands link up to the IP addresses from their documentation.