Not a happy Ex customer - How do I escalate any problem/issues/concerns?

I had persistent repeatable problems with COX services and finally after 3 Years I discontinued it. The bigger concern was whenever I get someone on the phone or chat I get someone who follows a script and refuses to elevate the problem. Terrible experience.
Here is a timeline to go with
Payed my last bill - 20 Feb 2020
Stopped using COX services - March 2020
Cox discontinued providing internet services - March 2020
COX discontinued my account - May 2020

Now, after discontinuing I am charged with a hefty payment of $209.44, I have raised my concerns with your customer execs to help me understand what I am charged for???
But they don't have any clear answer, only thing they say is - "You are charged till May 2020",
How?? Cox has stopped my services from March 2020?? No Answer...

On top of that - Support staff don't escalate this issue to anyone, who can explain..
So irritating and frustrating.

I was thinking to move back to Cox but now I am totally dissatisfied.