Hey, tomseppi! That's super frustrating -- your unlocked phone was working fine on the T-Mobile network for four months and then it suddenly stopped? The SIM troubleshooting was on point; but since that didn't solve it, I would have recommended the network check-in with our Tech team as well (kudos to you for moving on that on your own!). Have you had a chance to reach back out to see what the results of that service inquiry were?
Lastly, have you tried a master reset and then manually inputting the Generic T-Mobile data settings again? It's strange that all other functionality is working except for incoming calls; so I'm thinking perhaps a setting somewhere is to blame. Please let us know if you've already walked through that, and if you've had chance to check back in on your ticket with our engineers. Thanks!