Escalation for credit/refund request

We've been working on a credit/refund request for nearly two years now. The request is for multiple unused, unrequested SIM cards that were activated and shipped AT&T and resulted in several thousand dollars in charges. The error went unnoticed for several months due to our inability to access online billing which was further delayed by our AT&T representative not supporting our request to be given access to our online account and our assigned representative being changed.

9 months ago we provided a complete breakdown of the excess charges which was submitted for a credit/refund request and denied because 1) it was not submitted with proof there was an error on AT&T's part and that we had requested the cancellations, 2) the length of time the request went back even though this is largely because of AT&T's delay. We immediately followed up with evidence from AT&T's representative confirming their error, the lack of response from AT&T's representative in confirming the error was corrected and to allow us access to online billing so that we may confirm the error was corrected, and proof that we provided timely notice to cancel and requested updates that received no response from our AT&T representative.

It has now been 1.5 months since I've received an update from our representative and I've been emailing them weekly requesting an update and offering to provide any additional information or to meet with whoever we need to meet with to address this more efficiently. Since our representative has become unresponsive, is there an escalation path that we can pursue?