Missing promotion discount

On 8/23/26, I went to the closest Xfinity store to upgrade a phone (two hour drive). I purchased a new iPhone 17 with a current advertised promotion $400 discount for paying the cost of the phone over 3 years with Xfinity mobile service. When my first statement arrived, the promotion was not applied. Using the phone number that I thought was associated with the store where I purchased the phone, I called Xfinity on 9/5/26. The agent on the phone told me they would apply the credit. Since then, I have called Xfinity on 9/15/26 (the only call that resulted in a credit to my statement and payment for one month of the promotion), 10/27/26, and 11/14/26. On the 11/14 call, the second agent that I spoke to said there was no record of the promotion. I was transferred to a manager that said there was nothing that could be done and I must go to the actual store to work out this problem. How can I resolve this billing issue, as I am unable to go to the physical store.