Overcharged for a year

I signed up for internet at 30 dollars a month a year ago. I’ve been charged 50 dollars a month the whole time. Every couple of months, I get on the phone or go to the store, and the customer service rep agrees that I am being overcharged, and says the changes will take place on my next bill. Never happens. Then, this month, my bill went up five bucks instead of down 20. This is the company that says they will never raise your rates. I really don’t get it.