I too had DTac sim card. That isn’t the problem.
i bought a new iphone last year in the states and downloaded tge k-app and it wouldn’t work on tge bew phone. So, i tried to go back to tge old phone, but aftertrying on the new phone, the k app blocked me until i was able to return back to thailand. I currently using my Verizon usa sim and works fine with my k app on the mobile. It’s not about tge sim…it’s more to do with the original k app that is registered and usable on the phone that was given permission to log in. Once you switch phones…if you try and switch phones without the assistance from a k bank rep…you will be locked-out until you can return back yo Thailand