Some of us are talking about different problems here.
From what I can tell, there are at least two different server connections you need for this game to fully work.
- MSFS multiplayer servers: Live traffic, airport activity, flying with friends. You can see what server you're connected to by clicking your name at the top right and looking at the ping. When this doesn't work, you see NO OTHER PLANES IN THE SKY. Currently support is very unhelpful about how to resolve this issue.
- Xbox Live servers: Online status, Marketplace, etc. This is a separate connection, and issues are often solved by signing in and signing out. This doesn't fix #1!