Does anyone know how to actually file a formal complaint?

I’ve been having cell service with AT&T for years. February 9th I believe, I went to the service center and two men there told me I would get $600 in rewards if I signed up for home internet, because I already have mobile. I signed up for service and was set up for installation February 11th. The agent at the store initially told me I would get my rewards a week after install. After several calls to AT&T,speaking to supervisors and being transferred from one department to another, I found out that was a lie. They told me I had to wait 6weeks. I waited the 6 weeks *which was Saturday•, still no rewards. I called this morning and they are telling me it’s only a $200 reward and it hasn’t been sent. I want to file a complaint, but all I get is agents asking me to continue to repeat myself. I have a video recording of the agent at the store lying, but they won’t take it! Does anyone know where I can actually file a complaint that will be handled?