Try setting “Others can communicate with voice, text or invites” to everyone.
I loaded MSFS up and opened my Xbox account on a browser. I then started making changes to my permissions, Submitting the setting, then going back to MSFS and clicking on the Multiplayer settings again. It seems MSFS checks your account online at that time and if your settings are OK it will unlock the MP options. For me, it was setting “Others can communicate with voice, text or invites” to everyone that unlocked the MP settings.