Let’s not go that route just yet. 
Sorry, I must have missed that point about the mobile hotspot test. So the issue does seem to be on the computer itself, rather than your connection…
The only times I’ve had issues like you describe, along with a related issue of not being able to access the Marketplace are:
- Xbox Live login issue. Logging out, and back in again tends to solve that.
- Loss of MSFS digital license. Solved by doing step 1.
- Having the wrong version of the sim. For some reason I had one instance where the sim launched but failed to launch the sim update process. No online functionality worked at all, not even the Marketplace. I had to force an update of the sim from the Xbox Companion app, that predated the XBox app, and once the sim was updated, all online functionality was restored.
How did you buy the sim, MS Store or Steam?
Also, do you have access to another computer, even temporarily. It’s a big ask, but you could try installing a second copy on another machine, and see if the issue follows to the new computer.