RVGL uses P2P by default. Usually, the game uses a random port. However, if there is no port the game can use, problems can occur. Mostly others can't see you or you can't see others.
This can be fixed by opening the port 2310 in your router (a guide can be found here) and making a change in your rvgl.ini
Open your Re-Volt folder and navigate to the profiles folder. In this folder, you should see rvgl.ini. Open it and look for the Network section:
[Network] HostComputer = "" LocalPort = 0
Change it to
[Network] HostComputer = "" LocalPort = 2310
and save the file.
Forwarding port 2310 and changing the LocalPort setting in your rvgl.ini should solve this problem.