did you try and make a voice call?
did you make sure to disable VoLTE ?
can you test sprint and verizon?
if the phone lacks united states cdma like apple claims, then putting in a verizon pre-activated sim card, will basically turn the iPhone into a iPad cellular. and i think as long as VoLTE is working, it might give you the impression that the iPhone is compatible. so you need to make a regular voice call using cdma to make sure
apple is saying that this model does not have american CDMA and it only has Chinese Cdma. so i am still skeptical