Phones lost in the mail during ATT Next upgrade process

I've spent so much time (5 months) trying to resolve an issue with a lost phone as part of an ATT Next upgrade.  I followed all of ATTs directions in getting the old phone to them in November of 2017 only to have it disappear in the hands of the USPS.  We are now in April and in all that time I have not been able to get the issue resolved.  Meanwhile I am accruing late fees and getting my phone disconnected for not paying the nearly $500 price tag of the missing phone.  Anyone have some advice on how to elevate the issue or find some resolution?

I've spent hours (between 4-5 hours total) on the phone with 6 or 7 customer service reps from November 2017 until now (April 2018) and continue to get the assurance that it will be resolved only to discover that the credit requests are rejected and no one from ATT bothers to email, call, text, etc.  I only discover these things as part of my now regularly scheduled follow -up every few weeks.  A rep told me in early April that my phone was found and to wait the usual 24-48 hours.  When I didn't see the credit on my account, I spoke with a rep this weekend who had his manager take the phone after I requested that the matter get escalated (turns out phone not found).  My wife and I are convinced it was the same person pretending to be a manager (wife was listening in).  I'm starting to get the feeling that I am simply wanted off the phone since there is no resolution in sight.  I tried one last time today to resolve the issue using the ATT chat feature and was told to wait 3-5 days for a call.  

Just looking for some advice before escalating matters even further (arbitration and beyond).  It seems ludicrous that customers would be punished for following ATTs return policies.  Been a customer for over 20 years and have never had anything like this issue (then again, I've never really had to deal with customer service so maybe this is par for the course?).  Was a big fan of uverse, wifi, cell phone service, etc. before this.  May be leaving after this blunder.