phone not allowed MM#6, single owner for 3 years

Bought an AT&T Samsung Galaxy S7 at Best Buy in Austin Texas, paid for it in full over 2 years, unlocked it through AT&T, changed providers and it worked. I have used it in Texas for 3 years.

3 months after switching I am now abroad trying to use the phone under a different carrier and I run into phone not allowed MM#6, no matter what SIM card I put in it, same error pops up.

I attempted to solve this over the phone with AT&T and they of course assigned me the following case number: CASE: CM20190823_142710191

I received their reply quite quickly, 24 hours, and their solution was a copy/paste of the original phone unlock instructions but without an unlock code (don't need it, I have the old one provided by at&t I guess)

of course, whenever I reset my phone and remove/include a non-AT&T SIM card, there is no prompt to unlock anything, ONLY a "phone not allowed MM#6" error. No matter what I try to do, I cannot escape the MM#6 error.

This is an urgent issue, I need my phone to work asap and I have not done any weird things to it, always sticking to the correct process of unlocking through AT&T.

Please help me out here. Thanks.