Mobile device promotion

In July 2023 I saw on my Xfinity app that I was eligible for a new phone promotion offered if I traded in my current iPhone.  The offer stated that current customers were eligible. When I entered in my phone being traded the app  showed I would receive $700 off on an iPhone 13 if I traded in my wife’s iPhone XR. It showed I would get a credit each month of approximately $30 off the device payment provided I remained with Xfinity for two years and thus save $700 on the device.  I went into an Xfinity store in Austell, GA to get the phone where I was assured I was eligible. I was shown on a tablet what my bill each month would now be with the credit applied which added less than $10 to my current $64 bill. When I received my next bill, it included a $30 charge for the new phone with no credit applied. I received an $80 trade in check for the iPhone XR  in the mail but none of the $700 promotional credits.  I have since called customer service many times and gone to their store multiple times to get the error corrected. Everyone assured me each time that they had resolved it and it would show up on my next statement with a credit received for my overpayments already made. This has continued now for over three months with no credit to my bill. I finally reached  a supervisor in their billing department and asked if I could stop paying the extra $30 dollars each month since it was an error and she assured me my account would then immediately be sent to collections. I only upgraded because I was offered a promotion. It’s starting to look like I will never get the promotion I was offered. What do I need to do to get this corrected?