Poor and Unreliable Service and Connection with penalty fees payable?

The service we receive from MWeb is not up to standard. We haven't used the fibre since March, we were getting connectivity errors but every time we phone in to have it sorted it will work for a few seconds and then we were told the IPS address has changed, no luck in getting it fixed or sorted. I have now asked for the product to be cancelled but they want me to pay a penalty fee for early cancellation. How can I pay for a service that we don't get and why must I pay a penalty for a service that does not work?