Originally posted by Steve:

I've had connection issues in the past and this is what helped:

(1) Make sure both server and game are updated.
(2) Try turning on CrossPlay if it isn't, and have your friend (re)send the join code.
(3) Delete your old quicklink in the join menu and replace with the new one you make via the crossplay code.

This was the simplest fix for us. Hope it helps.