I am being given the run-around regarding device trade-in

I ordered a new phone only because of a $750 trade in credit promotion. I sent the old device, activated the new one. 

Trade in credit was never applied. 

I have spoken to agents/representatives 5 or 6 times about this, and every time they confirmed the device was received and credit will be applied on the next cycle. I have screenshot this each time. And yet every month, every cycle, the bill is the same. I feel as though I am being stolen from, what am I supposed to do?