The game relies heavily on Steamworks. Ensure your Steam overlay is active to accept invites easily.
The gold standard for stability. You can rent these from providers or host your own using the GROUND BRANCH Dedicated Server tool on Steam. GROUND BRANCH over the network
If you are hosting, you must forward UDP Port 7777 (Game) and UDP Port 27015 (Query) in your router settings. The game relies heavily on Steamworks