Att scammed me out of my trade in phone. Theft

I purchased an iPhone 14 pro Max with the trade-in option. With an $800 credit. AT&T never sent me a return kit. So I called them and had them send me the kit. I sent them the trade-in phone the day after I received the kit. That was 5 months ago. I called and spoke with someone a couple of months ago and they told me that I would just need to keep waiting for my credit. So I did. And now a total of 5 months later I called them again and now they are telling me that my trading was received late and I will not be getting a credit. But I will also not be getting my phone back. They offered me $210 for a phone that I could have sold on my own and got $400 for it. I have been a customer with AT&T for over 20 years. And if this doesn't get resolved I will be moving to verizon. The rep let me talk to his manager. And the manager told me that the $800 credit was above her limit so she sent it to someone else. I'm waiting to see if they approve it. The AT&T rep even told me he agreed with me that AT&T was in the wrong. Because they never sent me the kit. And if the day that they sent me the kit it was past the 30 days they should have told me that I'm sending a phone in for nothing. They should have told me that I'm not going to get the $800 that I was promised.. Instead they let me send it in, thinking I would get it, and kept my phone. This is theft. They agreed that they received the phone and it was in good normal condition. The tracking number shows it was sent the day after I received the label. They agree to this as well. I have all of our conversations recorded. I have screen shots showing the device has been received by att and approved. If this isn't resolved and my account credited by April 18 (the date the "manager" said to wait) I will be filing an FCC or/and BBB complaint.