Agent Offered Me a Promo... and didn't receive it.

So on 8/11, I was going to cancel my service due to cheaper and faster services elsewhere.

After talking to Sophia, on the retention team, they offered me a 40% off discount off my current plan and I accepted this offer with written agreement.

A month went by and I double-checked and the offer was never applied to my account. I talked to another representative on the Loyalty team today who overlooked my chat logs and said they couldn't offer this promotion... anymore and ended up cancelling my service.

What do I do? Do I file a report with the FCC or credit card dispute?