How to escalate to someone who actually understands billing issues

Since signing up my new contract last November I've had a consistent billing problem every month. The contract was at $20/month but I keep getting billed $50, without applying a $30 mobile discount to my internet bill. The contract I signed up is an internet + mobile bundle plan. The regular price for the internet service is $50, but I switched my mobile to Xfinity Mobile line, and I'm supposed to get $30 mobile discount for my internet bill every month. So my internet bill should always be $20/month.

In the past 6 months I've been talking to probably 50+ Live Chat Agents. Every time they admitted that it's a billing issue in the system, and they would apply a $30 discount for my bill in that month, and promised me the issue would be fixed in the next month bill. But 6 months passed, the issue is still the same - I'm still getting $50 bill every month.

The Customer Support people I'm talking to are not dealing with whatever the underlying problem is. Every time I have to spent 3-4 hours to talk to 10+ agents because they keep transferring me between different departments, such as internet billing, mobile billing, etc. The mobile billing team has no way to fix this issue because they don't have access to my internet account. This has been going on for 6 months now. It's an endless stream of promises that are broken and explanations that don't explain anything. How do I reach someone who is at a level of support that they actually understand what they're doing, and can set things up so that we actually get billed the amount that we've been promised over and over we would be billed? It is unconscionable that we have been made specific promises by Xfinity CS and these promises are broken again almost as soon as they're made.