Illegal price increase mid contract

I have been using Comcast internet for over 10years, and a few months ago I was offered a promotion for 35 dollars per month for 800 Mbps internet with the price guaranteed for 12 months via contract. This month, my bill suddenly increased from 35 dollars to 129 dollars and when I contacted Comcast they said that the previous agent in January gave me a discount incorrectly and thus they are now increasing prices. It’s not my fault that the previous agent screwed up, and now I am being asked to foot the bill. I asked to cancel my contract and switch to a competitor, but they are making me pay an early termination fee. If Comcast can’t keep their side of the contract by maintaining the promised price, why should I need to keep my side? All I want is to cancel my service because of the illegal price increase mid contract.