I have AT&T as sim1 and T-Mobile as sim2 on my Lumia 950 dual sim and had set my cellular data connection to use data from T-Mobile. Since Jan 6th my at&t sim wasn't connecting to the network ("no service"). I tired changing my cellular data to use at&t. That's when it started connecting to the at&t cellular network. But if I go back and change my cellular data to use tmo, my tmo network works but att sim says 'no service'!! Tried restarting my device multiple times. No use. Tried putting both my sims on an android dual sim phone, same behavior on that device too! That tells me that the issue is not with my Lumia. Any ideas on why my att sim says "no service" if I set my cellular data to use tmo?.