Hi guys. I did it, finally!
I had to do multiple tests, including portforwarding
all UDP traffic to my PC, to be able to analyze it using Wireshark. This way, I discovered that, after the first tree packets (my server heartbeat, master server response, my server response -on ports 27960 & 27950-), then more packets are exchanged, and they come from more IP addresses and UDP ports.
My router changed my source port for the first outgoing packets, and those later packets pointed to the modifed port, that was then closed or simply not controlled by OpenArena. To have a server appear on the server list, these additional packets must work.
To fix, I had to enable an option called
"Corresponding NAT entry for outgoing connection" in my "port forwarding" setting. This way, my first outgoing packets exit from my router with 27960 instead of a random port, so later incoming packets point to the right port!
I had already enabled that option during previous tests, but at that time it didn't work at all cause of some other configuration error that I later fixed...Do you know other names for that important option under different router models?