Samsung Note 8 migrated from Verizon to AT&T - Won't stay on LTE

I brought my very new Samsung Note 8 phone over to AT&T about three months ago.  For three weeks, everything was working great.  4G LTE all the time.

Then it started dropping down to "H" and "H+", out of LTE, and staying there until I restart the phone.  After restart, it returns to LTE for anywhere from 30 minutes to over 12 hours, and then drops down again.

The phone is unlocked and fully released from Verizon.  We have tried replacing the SIM at the corporate store, which didn't change the symptoms.

I'm trying to understand root cause, and looking for possible resolutions (short of selling the phone at a huge loss.)  It's a Verizon build (with their add-on software I the factory build), so I suspect that is central to this, but what can I do to mitigate that?

Thanks for any help!

Tim