AT&T customer service reps dishonest...

I am so fed up with AT&T. Long story short the new iphone was fixing to come out and I wanted to upgrade. So I called AT&T first to make sure I would not have to continue to pay for my phone, and a new phone if I upgraded, I told them the phone I wanted to get and they had my account pulled up. I was assured if I upgraded I would send my old phone in and not have to pay anymore for it. So, I upgraded, paid the full amount for a new XS Max. Got the trade in kit in the mail like I was told and sent my 7+ back. After they received the phone, and it was looked over I received a small refund back for it. 2 months go by and I am still being charged the installment, so I call. The rep assures me this is the last month I will be charged for it. The next bill comes around and still the installment is on there, so I call again and they open a case on this. I never hear back in the time the said they'd contact me, so I call again, am told they need another week. Week goes by and I never heard from them so I call yet again, this time I'm transferred to the loyalty team where I am told they are going to resolve this. So they need several days to look into this, well I never hear back from them either, so I call again. I am very frustrated at this point and am told I will get a resolution on the phone during this call. After looking into the matter and getting supervisor approval they tell me they are going to credit my account with the amount of the installments. I get a reference number to confirm this and am asked to give them 4-8 business days for it to post to my account. It has now been 10 days and still I see no credit. So I talk to AT&T again and am told they cant even find anything on that reference number, or any notes on my account. They said they saw an investigation that was turned down due to lack of evidence of me sending the phone back. I am furious at this point, I have repeatedly been lied to by customer service reps along this whole process. It seems like they just tell me anything that sounds good to get me off the phone... What can I do about this? I told them during one of the conversations I am not paying anymore on my bill until this is resolved, and am now approaching 2 billing cycles where I have not paid and my account is negative...