Unlocked Samsung G10+ phone "not compatible" with Xfinity Mobile but the same AT&T-locked phone is "says the computer" based on IMEIs

I have two Samsung Galaxy S10+ phones, purchased directly from Samsung. They are identical (HW, Android OS, etc.) except that one was purchased Unlocked (with no AT&T bloatware) and the other AT&T Locked. Both have been used with AT&T for the last few years. I now have the unlock code for the AT&T phone.
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I would like to switch both phones to Xfinity Mobile. But when I try to do that, they tell me that IMEI code of the unlocked phone is "not compatible" with their network. On the otherhand they says that the IMEI code of the AT&T-locked phone is compatible.
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The only *subtle* difference is that Samsung gave the unlocked phone a slightly different Model Number for tracking purposes: 
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= Samsung Galaxy S10+ unlocked phone
Model Name: SM-G975U
Model Number: SM-G975UCKEXAA
 "About phone" says Model number SM-G975U1
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= Samsung Galaxy S10+ AT&T-locked phone
Model Name: SM-G975U
Model Number: SM-G975UCKEATT
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Based on the IMEI lookup of a slightly different Model number, Xfinity says the Unlocked phone is not compatible with their network. However I checked with Verizon and, based on IMEIs, they say *both* phones are *fully compatible* with their network. Xfinity Mobile uses the Verizon network so both phones are *also* fully compatible Xfinity's network, despite Xfinity's "the computer says no" stand.
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I've tried escalating this up to the VP of Customer Experience. He handed me off to an Executive Resolution Office who contacted me and just repeated the tired "the computer says it's not compatible, there's nothing we can do." There is -- turn off "bureaucracy mode" and have somebody technical look into it and add the unlocked Model number to the list of compatible phones. Then "the computer" would say the right thing. AT&T and Verizon have had no problem doing that so apparently they want my business more than Xfinity.