The iPhone XR model 1984 is the only model sold in the US and is compatible with all of the providers. Do you happen to have a Public Beta or Developer IOS software version on the iPhone? I had that happen to me with a Public Beta version of IOS and Xfinity would not recognise it until it was restored to the current version.
Also if you purchased a new phone from Xfinity, it would work with other providers once it is unlocked.
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