I have the same issue.
We have bought the game 3 times.
Windows 10 are all 1903
XBox (Beta) is installed
Windows Firewall is disabled
AVG Firewall all temporarily disabled
My client is started in in game
Using the XBox (Beta) app to invite to game causes the started clients to be highlighted
The co-op menu in the other clients says that my game is joinable
Any attempt to join says 'Session not found'
The router fireall does not drop any packets
I can't find any log files?
There are no error messages?
How can I troubleshoot this?
We have been waiting for years for the multiplayer to work?