Trade In phone received without credit since November

I ported in my service to At&t in November 2019 and added 2 new lines with the trade in phone and new line for $500. Cary Town Center in NC provided the service and made an error on the additional line on my account. I noticed the missing credit in January and brought the issue to the store manager named David, which took a copy of the documents I brought. Then I was told they will call me in the next 2 weeks. 3 weeks after I followed up and spoke with Robert, then the issue was passed to Cartier the new manager of the store then 2-3 months still no resolution and the manager cannot provide me a proof that the store is working on it. Then Cartier asked me to send an email with all the details copying Madison, the store manager. Both Managers were incompetent, both did not acknowledge the email sent. I called the following week to follow up and spoke with Cartier and he told me he will call Madison and will call me the same day. NO CALLS, NO FOLLOW UP. THESE MANAGERS ARE THE WORST AND THE MOST INCOMPETENT MANAGERS TO WORK WITH. I NEED TO SPEAK TO SOMEONE AT AT&T WHO HAS AUTHORITY OVER THESE TWO SO CALLED STORE MANAGERS AND NOT INCOMPETENT AT THERE JOB.