I, too, have T-Mobile on a N20U. While it's not nearly as frequent as your issue seems to be, there have been plenty of times in the past I've had to restart my phone due to T-Mobile. It was much more common on my S8+ and S6, but not so much now.
I have the S21 Ultra. I had the same problem outside last week. I called TMobile and they had me restart my phone while they sent a message to it. I haven't had the problem anymore.