Trade In Device IPhone 11 to 14pro

Hi 

I have traded my IPhone 11 and got my IPhone 14pro max where there was a trade in offer 800$ off and the payment is up to 3 years I would get all credit back initially and I was happy too the mobile 

After 2nd month of use I received my bill and no credit value for old mobile ($25) added to bill then called AT&T and told them he informed me that your device which you traded is not that value I was shocked and told him to give my old one back and take this new he told NO 

then after multiple calls here comes the Manager on call explained everything what happened 

Then she told me that *The line holder has deactivated some old line that’s the reason you are will not get that credit off $799 off on you mobile* there is no point mentioned or while canceling aslo we confirmed would there be any problem the agent told no because you have 5lines in account which is good 

Then only we cancelled 

Now I am in confused state what to do 

I have no idea how to deal with it 

can someone help and let me know how would this work out and get my $799 credit back on my old device so that I can’t pay my full amount for new one without that trade in 

Its been 5th month now every month we call customer support and every time each person tell us some different story , as am new to USA I have no idea how to deal with