Just thought I would contribute my experience since I see that the online multiplayer is still in beta so I hope this helps.
I am on a Macbook Pro laptop on version 10.15.6 Catalina.
Went into the join a room menu for online multiplayer. My friend on a Windows computer had created a room with a name and a password. The screen for me just said that it could not connect to the internet. When I attempted to refresh the page the entire game crashed. Rebooted the game and the exact same thing happened again.
Hope this is helpful in your quest to create online multiplayer! Sounds like a difficult endeavor :)