Device credit not being applied after 6 months of asking monthly and being told they will fix it.

I traded in an iPhone 14 Pro seven months ago for in iphone 17 during a trade-in promotion and I was to receive $23.05 a month credit on my bill every month for 36 months but it has not happened. I have contacted support every month since February when the bill comes, and every month a different agent says they will fix it.  I have numerous support tickets and a very lengthy chat transcript, but no results.  They just try to sell me cell service to an Apple watch each time, which I do not want, yet they persist. It is very aggravating.  Does anyone have any idea how I can get a real person with knowledge and authority to actually correct this?