Trade in credit and premium upgrade credit issue, unreliable customer service

Early July 2025, looking to upgrade all lines on my mobile account. Found a great rate for trade in credit for 2 phones and a premium upgrade credit for the 3rd one. Went into the store and purchased 3 new phones and upgraded all lines to premium. Was told I would receive 600 for each trade in and 500 for the premium upgrade for the 3rd with no trade in required. Totaling 1700 divided over 24 months as credit on my bill. After the transaction that figure changed drastically, the amount wasn't near what I was quoted and agreed upon in person. I went back to the location for assistance and was told the system wasn't working properly and was giving problems to many customers regarding the credits. They told me they will put in a ticket and it will be resolved. I patiently waited and checked in for a few weeks with no resolution. I went back into the location and still was told the same thing, system not showing the accurate information and that if it wasn't resolved soon the manager assured me he would personally take care of it. Some time passed probably 2 billing cycles and i checked back only to find that the so called manager had left the location and now its October and I still haven't received any help fixing this. I'm paying way more than what I should be and i really can't afford it. I would have kept my old phones if I would have known this would happen. I'm about ready to contact the fcc and the bbb if this can't be remedied.