Hey,
When RenX does nothing when you press "Join" this normally means that the RenX Client is unable to establish any communication with the server (aka, it cannot connect to it), so this would point to a firewall issue.
The TCP Connection Closed is totally fine, that is just the game server stating that the TCP Connection to the master server has been complete (The master server is the server in which holds the server list, and sends the server list to clients upon request).
However, you say that it's on the same network - so this other player is also in the same building as you i assume?
Have you both tried to connect directly using the IP Address tool - reason is if you are hosting this from home you would not be able to use the server browser to connect to the server, reason: When the RenX server broadcasts to the master server to say "Hey, I'm a new server, add me!" - it will add itself from the Source IP Address that the request was sent from, in your state this would be the WAN side IP Address of your router.
But hey, I am starting to ramble on now haha.
If u have any more questions let me know!
ps - I am not a RenX Dev or anything, so some stuff I've said above could be wrong.
