I managed to get people to connect to my server by a really weird workaround
I had to use a Java program called Portmapper to force Upnp to work, as the game server somehow wouldnt bind to the forwarded ports any other way?
(Makes me wish the server had a UPNP option built in, tbh)
GitHub - kaklakariada/portmapper: A tool for managing port forwardings via UPnP
I can't explain why i needed it, as all my other port forwards work (including one to the same machine for a different program)
Edit: heres the log entry from my brother who couldnt connect with this fix... but the port used is totally wrong? (IP edited for privacy)
Server is set for 26900, but clients are connecting using a totally different range...
2021-12-10T20:54:49 29594.130 INF NET: LiteNetLib: Client disconnect from: 1**.1**.1**.1**:53052
Edit 2: I may also be an idiot and have simply typod a port forward, and the games trying to use the steam ports instead of the default ones
Due to two routers here using different syntax for entering the port forwards, i'd told it to forward 26900 to 26905, instead of forwarding all 5
*facepalm*