[2021] Multiplayer doesn't work properly

Hey, have you checked whether or not your/your friend's steam P2P port is open?
You can check them by following the steps:
https://www.windowscentral.com/how-open-port-windows-firewall
You should make sure that the UDP ports 3478,4379,4380 is open in order to have P2P connection with others.
Reference(Steamworks P2P Networking and Steam Voice Chat): https://support.steampowered.com/kb_article.php?ref=8571-GLVN-8711