Device credits not being applied after trade-in

I traded in an iPhone for an iPhone 17 a few months ago. The trade-in was accepted and the device credits were successful applying to the account each month. Suddenly, after a few months, they stopped applying and my bill jumped up. The customer service rep said that the line was not on an eligible plan so the credits stopped.

I think what actually happened was a credit was made to my account for a different issue and that accidentally accelerated the device payments in the system and caused the device agreement to become void. Please help!