Help with Issue, how do I escalate to someone at the Corporate Level?

In January if 2020 I went to a local Authorized user AT&T store to buy the Iphone 11 Pro and turn in my Iphone 7 for a $300 credit. Almost a year later I still don't have my credit. I have been into the store every month since to resolve this issue. The manager has given me the two possible tracking numbers the phone was included in and a trade-in receipt. Each time I go to the store the manager tells me she will call and email on her end to try and get this issue resolved but never follows up with me. Now when I call to ask for the manager they tell me she is not in, she never returns my calls/messages and even when she answers the phone she lies and says "the manager is not in." I have tried talking to the Promotions/Credit department and various other customer service departments and they say they have no record of ever receiving my phone and they are unable to open a case without some way of being able to locate the trade-in details. The promotions department told me that there was not even an Iphone 7 trade-in credit promotion in January of 2019 but the store office manager assures me there was a promotion in her system of upgrading to the unlimited plan for a $300 trade-in credit or they would not have been able to add it to my account. At this point, something is very fishy and I need this store investigated for fraud and scams! Perhaps they have a shady side business going on stealing people's phones for "trade-in" and not actually turning them in and selling them on the side for cash. This is the only explanation as to why the store keeps telling me they will help but never do. I have been with ATT for 18 years and never once had an issue with a phone trade-in credit until the first time I went to an Authorized user store and not an official corporate store.