Blatantly lied to and ripped off

I needed a phone with the ability to hold 2 sim cards from different carriers, so I could stop carrying 2 phones around.  After some research, S22 ultra can do it. I had Verizon, but they wouldn't cut me any deals on a new phone. Then AT&T shows up to change my modem and I tell the store manager (apparently they make house calls now?) what I'm looking for, he says he'll knock $800 off for my trade in, and I can pay it off in a month, to unlock it, so I can use 2 sims. It was made very clear that I was only switching for that reason.  After a month I see that the credit isn't showing up on my bill. I talk to the manager again, restated the whole reason I switched from Verizon, he says he'll handle it. Another month later, still not fixed, he ghosts me completely.  I call customer service and after being told that I am lying, then that I misunderstood,  I'm told that they won't do anything to help me. If I want to pay it off to unlock it, there's no credit for the trade in, and they won't return my old phone. I have text messages from the store manager that prove he told me that I could pay off the discounted price to unlock the phone. I get a customer service manager on the phone, and she won't even acknowledge that I got ripped off. All I can say is that I promise that this will cost them more to rip me off than to stand by their word. I'm canceling my internet and cell service. I'll pay off the phone, so I don't hurt my credit, but this is immoral at best, and most likely illegal.