How to escalate within AT&T about missing installimet credits

I was a long time AT&T customer (17+ years), left AT&T for brief period of 7 months and joined back AT&T again in Aug 8 2019 by signing up 5 wireless lines.  At the time I signed up (over the Internet & chat with Agent), AT&T was promoting a free iphone 8 (worth $600) by giving me $20 monthly credits for 30 months (i.e charge me $20 and credit back $20 every month, lock in for 30 months).

To date I have not received any credits.  I had been dealing with AT&T customer service/retention department for since Oct 2019. I estimated at least 20 calls, and each time spent almost 2 hours per call.  Still not resolved. To make it worst, with So many AT&T customer services that I spoke to, with the exception of one AT&T customer representative (who actually call me back twice), the reset of those AT&T representative that promised would call me back never keep their promised.

I have all the chat log on the day I signed up for service clearly indicate I was eligible, I also have my online order recoreds showing the promotion. I have all the notes I talked to all the customer representative since Oct 2019.  Who can I ask in AT&T (other than the customer royalty department or standard customer service dpt) to help me in this case?

thank you