After much testing, I can confirm the problem is definitely on his end but I am writing this on his behalf.

The problem first occurred a few weeks ago, after months of playing together all of a sudden no one could connect to his hosted games (timer runs down after clicking join). He tried the wormNAT2 module but when he tried to connect to the lobby (before selecting the lobby) he got an error message.
He removed the wormNAT2 module and he was suddenly able to host without any issues

Last week, the problem returned and no one can connect to his games again.
I don't understand what could trigger this when apparently nothing has changed on his end, besides some sort of automatic port blocking happening through his ISP or router.

Any ideas?

I asked on the Worms Armageddon Discord server for you, got this reply:



If you want to join the Discord server - https://discord.gg/faVMcU

Next time I see him I'll try the port checker and the rest, and if we still can't get it to work I'll get him to try the discord.

Very helpful response, thanks