Repeating the stuff I've written in my PR here: fosrl/docs#66
There is no bug template so I just summarised my problem and solution instead, this applies mostly to Unraid as it's "set in stone", but could also apply to regular Docker Compose users I suppose?

I have debugged an issue for 10-15 cumulative hours and I finally found the solution to my problem. The idea came to mind from reading fosrl/pangolin#227 (comment). Apparently the Gerbil Docker container internally does not like having different internal and external ports, it wants them to be matched.

I tried port 51820, 51824, 1194, all of them responded to tcpdump -i any -n udp port [PORT] tests (confirming UDP packets were reaching the server when testing with echo "poke" | nc -u [IP] [PORT]), but I still had no Newt containers being able to connect.

My 51822 port was busy by another user on my network so that's why it took me so long to try the default setting, but once I did everything instantly worked.

The issue was so obscure that even with great Discord support (thanks Astral), it was very hard to identify. For now, the earliest and easiest thing to do is at least notify any future user where an issue might occur, until it is hopefully fixed in the system itself later on.