Carrier lock

My phone iPhone 11 Pro says carrier locked. It’s paid off and I took

it to spectrum where it was purchased to have it unlocked and the customer service guy says that spectrum does not have it locked . Only carrier Thais be used with phone and I do not know account number associated with old account . I switched during covid due to all of AT&T stores were closed and needed a new phone quickly for work . I purchased and subscribed to spectrum and paid off the phone and account . I still have their internet and tv . I got a new phone for being new customer going back ro AT&T and was keeping this phone for spare . I have been in store and on phone for house trying to get it resolved . Can anyone help ? Apple did issue new phone some time after initial purchase due to issue covered under Warrenty . I can not figure out how to get it unlocked.