Mint mobile has locked my phone to T mobile

I paid cash for an unlocked iPhone 11 at Best Buy... I was using Mint Mobile at the time. I’m switching to AT&T now and have come to find out the Mint SIM card locked my unlocked phone to T Mobile... the carrier Mint piggy backs on. Now I can’t unlock my phone to move forward with AT&T... Mint has cancelled my account and claim they can’t do anything about my phone because ‘they don’t lock phones’. I went to T Mobile and they agree the phone is locked in their system but can’t unlock it due to me never having had a T Mobile account ever. I spoke to a manager and they are supposedly looking into this and will call me back.... could take 3-5 days... HELP!