Same situation here. Utterly infuriating and no issue being online with any other game on the Xbox X.
Reset the game, port forwarding, static IP tried to toggle the Data settings, joined my iPhone hotspot to get through authentication - pretty much everything.
Marketplace constantly greyed out then all of a sudden, it will be online again for maybe an hour or two and then inexplicably go back offline.
Sometimes I start the game with it showing online then a few seconds later it tells me I’m offline.
I cannot even fly offline. I have the offline files installed but won’t let me fly as it believes they are NOT installed, then takes me to the shop to install them and it says installed.
Total joke of a situation.