Misrepresentation of promotion. Where can I report this?

I purchased two iphone 8 devices (buy one get one free offer) and signed up for AT&T wireless service as new customer. I was promised by the salesman that I can get $200 + $500 credit. I was told that these two credits will be shown up in my account in 1-2 months separately. However, after I receive my first bill statement and brought it back to the store, the salesman told me a different story. He said I will get $200 credit only, but not another $500 credit. According to the salesman, I will get $500 in total if I bring in 5 phone lines into AT&T. Since I only have 2 lines, I will get $200 only (so far I have not got it yet). He said that it is me who misunderstood him in the first place. While I was talking to the salesman, the store manager came out. He concurred what the salesman said and insisted that I misunderstood them. I am certain that the salesman did not mention about 5 lines at all when he tried to get me sign up. I remember it clearly, and I even discussed this offer with my family before making the decision. When signing up me for the new service, the salesman did not mention that he signed me with business account neither.

I argue that the salesman (and maybe the store manager together) is lying to me. They played tricks and gimmicks with promise, but denied what they said after the account is signed up. This AT&T store is located in the second story of Burlington mall in Burlington MA. The way they cheat me to sign up the account damages both customer's benefit and AT&T's reputation. Therefore, I want to report this incident to AT&T. I wonder what channel I can use to seek for fair solution?