Same thing I did. Switched to Tmobile when ATT screwed me over the iPhone X.
I switched from Verizon to ATT years ago like that. I had alltel for like 15 years, and when Verizon bought them, they gave me the run around in the store. The manager called corporate for me, and they wouldnt do anything to appease me. So I told them to cancel everything, handed them my brand new phones, and went across the street to ATT, and spend like 1500 bucks on 2 new phones. Walked back to the verizon store, and showed the manager "see, its not about the money, its just customer service. You would rather have a new customer than a loyal one."
I went to Tmobile a few years ago, because ATT was so expensive, and their service sucked in our neighborhood.