Tmobile lost my trade in and is not honoring trade credit.What can I do?

I am dealing with the same thing. My iphone 11 Max Pro was supposed to get an $800 for trade in. They said they can not locate the phone even though I have tracking number saying it was delivered, etc. I have had so many issues since I made the switch from Verizon on May 31, 2023 and have spent hours and hours on the phone trying to straighten so many issues. I seriously thought someone was playing a joke. I can not even explain how much time I have spent and I am so angry beyond belief. I was with Verizon for many many years and no issues - I should have stayed with Verizon, but I was trying to save a few bucks. I am getting completely screwed and do not know what to do. Is this happening to others? I am so angry that they are doing this and getting away with it. It is so unethical. Has anyone gotten a lawyer? This is so wrong.