Need to unlock iPhone SE

This might be a little long.

I originally purchased an unlocked iPhone SE from UAE in 2017.

In December of 2018, I took the phone to Best Buy in Fargo, ND to have the battery replaced.

The staff at Best Buy told me that while he attempted to open the phone a cable was broken and that the phone needed to be replaced.

Best Buy requested a new phone from Apple for me.

I received the new phone from Best Buy and put an AT&T SIM in and used it normally.

However, when I went to Thailand in January 2019, I found that the phone is locked.

So, when I came back to Fargo, ND, I took the phone to AT&T shop.

The staff at AT&T said they couldn't unlocked the phone, since it did not appear on AT&T record and recommended me to contact Apple directly for support.

Please let me know how this issue can be resolved.

Thank you very much.

Pat