Docker installation works, Ubuntu installation doesn't

Hi guys, I have a problem which I can’t solve by myself. I got the docker version of Jitsi running on my Ubuntu virtual Machine just fine, I’ve tested it with multiple users and everything is working. Now I wanted to add some custom features (rooms still exist after all users leave, moderators can unmute participants) which I found tutorials to. These custom features all use the non-docker version so I tried to install that. I followed the official tutorial und opened all the relevant ports.
The installation was successful, I can even go to the Docker website on my server and open a room. If I connect with another user, I can’t see video or hear audio. The chat doesn’t work either. After some time an error message pops up and says, I was disconnected.

In the jvb logfile it states, ‘Address discovery through STUN failed’. I followed the ‘behind NAT’ section of the tutorial, as that seems to fix this problem for other users, but I didn’t work for me.

I don’t understand, why it doesn’t work even though the docker version runs just fine.

Any input would be much appreciated.