upgrade when wrong?

We recently upgraded two iPhone 13s to iPhone 16s through an online trade-in promotion with T-Mobile, submitted on 9/13/24. For the first month, our October bill looked fine, and the trade-in credits were applied. However, on the November bill, one of the credits was unexpectedly removed, leading to a more than $40 increase. When I contacted customer support, I was informed that the trade-in promotion was removed because our current rate plan supposedly doesn't qualify. Now, T-Mobile is billing us full retail for the phone unless we switch to a more expensive plan.

This feels unfair, as we weren't informed of any rate plan restrictions at the time of the upgrade. Now, they're asking us to either pay more each month or lose out on the promotion we signed up for. At this point, I either need T-Mobile to honor the original deal or return my old iPhone. This was a frustrating experience, and it feels like we were misled.

I don't want to change my plan.  ¿Cuáles son mis opciones?