Never received full credit for trade-in I was promised after being escalated and approved by Vincent at AT&T Help Forum

About a year or so ago I was very disgruntled about not receiving proper credit for a trade-in. I posted to forums about feeling scammed by AT&T as a brand new customer. Vincent, an At&t employee, helped me then and investigated the claim thoroughly. He found that I should have indeed received $800 in credit for the trade-in promotion that was advertised at the time I signed. His last message to me on the forum was to confirm that the $800 credit was approved and I would see the credits result on my bill. And I had hoped he was correct - however...

I just started to look at upgrading my phone and wanted to see how much I still owed...because it seemed my bill really didn't get reduced as it should have Vincent had verified I was indeed owed $800 credit for my trade-in. I know a lot of time has passed, but when you have an auto-pay bill, sometimes you just have time to notice these things until you really need to!  
Once I started looking at doing an upgrade, I noticed I have only been getting $11.67 credits on each bill for my trade-in. It says I have 30 of these credits for a Smartphone Trade-in Promo, however that only equals $350.10 and does not equal the $800 I was promised and that he had verified would take place after working with you last year. 
It still shows I owe $293.93 - but this cannot be the case if I am due an $800 credit. In other words, this credit NEVER was fixed STILL as it should have been. I am missing $450 in credits that I am still owed. Why do I keep seeing these type of issues happening for so many customers for AT&T - it seems they never live up to their word???? Here's the breakdown for my trade-in on my bill, which is not correct if I were to receive an $800 credit. The amount financed is showing $801.99 - and I was supposed to receive $800 for my trade-in, something is absolutely still not correct here and I will definitely be looking to switch to a more honest company once my contract ends. This is sooo disheartening. I need someone to fix this credit amount I was promised and that I am still owed to reflect the FULL $800 credit was to receive for my trade-in promotion. Please see my forum history post for the full story and interactions with Vincent.
From my bill: 
APPLE256GB 1600000********** (numbers omitted for privacy)
Established on Oct 24, 2020
Equipment price $1,049.99
Equipment down payment $248.00
Amount financed $801.99
Installment 19 of 30
(Apr 24, 2022)
$26.74
Balance remaining after
current installment
$293.93