Repeatedly charged incorrectly

I live in the US and my husband & I are both on a plan with AT&T Wireless (unlimited data and texting) with two phones and one apple watch. We have been desperately trying to work through the situation I will describe below to no avail.

About one year ago, we purchased two iPhone 8+ for the BOGO deal. Recently, we both purchased two iPhones- the XS Max. I wanted to unlock one of my phones so that a family member can use it as we continued making payments on our plan. My husband called AT&T and was told that if we paid roughly $399, then the phone would be unlocked. Unfortunately, we later learned that the wrong phone was unlocked. So, we called customer service back and confirmed we would receive a refund and that we should try again and make sure we indicated the correct IMEI number the second time to ensure the right phone is unlocked. We got the refund a few days later. However, AT&T decided to add back the $399 on our account, showing that $399 was past due. When we called AT&T customer service, we reached someone who removed the $399 charge (to my understanding this was because the other phone was still unlocked and needed to be locked again with the installment plan reinstated) and said if we paid $366 (this was after one billing cycle had passed, so the $399 was reduced) then we would get the correct phone unlocked. My husband made a payment over the phone. We have a receipt for this payment. Then, just a day or two later, we received an email saying that unfortunately, the phone could not be unlocked. My husband called to ask for a refund, and we were told to wait 2 business days and we would receive. Well, it's been 2 business days and not only have we not received a refund, but an additional $366 charge was added to our account. The account shows we have $366 past due. This has been an ongoing issue for a couple weeks, requiring one of us to get on the phone immediately after getting home from work just so we could spend the few hours of downtime we have each day failing to resolve the issue. In total, we must have spent roughly 7 or 8 hours on the phone trying to explain this issue. When we got cut off after finally reaching someone who seemed like he could help, the line got disconnected and he never called us back despite verifying our number in case of disconnection.

Why is AT&T charging us money repeatedly? All we want is a refund- I no longer wish to unlock any phone. In fact, after this experience, I believe I will be canceling our internet, DirecTV, and wireless bills with AT&T. Never have I ever been more frustrated with no results to show for all the hours I have put in.