Account refund and poor customer service.

I cancelled an account when I was transferred out of the country in December of 2017. I returned all the boxes to the local AT&T store in Houston. . I have now discovered that ATT has continued to withdraw the monthly fee out of my checking account for over 2 years. I had gone paperless on billing so was not aware they were still charging me for this. I called this week to cancel the old account and asked for a refund since the home was sold and have not used any cable service at the old address in Houston since we sold the home. The first account rep was extremely helpful, agreed there had been no activity on the account other than what appeared to be automated monitoring tests from ATT. She also advised her supervisor would contact me to work with me on a refund (which is for almost $7,000). He never called. Now the new account rep says there is activity on the account and she can give me a 2 month refund. That is totally unacceptable since we cancelled the service and returned the cable boxes in December of 2017. She passed the problem to a supervisor named Mandy - Employee ID (Edited per community guidelines) who refused to provide any refund. She also would not put me through to a more senior executive that has the authority to correct this gross error. I have been a loyal ATT uverse customer. In our new home, We have 8 televisions and 1gig wifi with uverse - we also have 2 cell phones and 2 ipads also using ATT services. I plan to start looking at a new service provider immediately given this refusal to provide a refund for the 29 months of service billings where there was no activity on the account. She did suggest I write to someone in the corp office. Who should I address this complaint to?