Usually it's best to "cancel" the contract immediately after signing, then when the cancellation comes in effect a year later it's easy to get the new customer rate again.
Or if you don't mind the hassle and have another provider available at your location, switch to it via some broker like check24 and get it even cheaper. I'm currently on 30/6 mbits for 11€ for two years, before that I had 120/10 mbits for 17€ over two years. They would turn into 25€ and 35€ after two years.
It is kind of ridiculous how they actually punish loyal customers by making them pay more after two years if you don't do anything. Sure, enough people are lazy enough to make this model feasible, but just imagine the butcher you've been getting your meat at for years suddenly goes like "hey you come here all the time so I'm gonna charge you 30% more"