Home Internet uncapped - activate one of these contracts at your own PERIL

Home Internet Uncapped - a MONTH TO MONTH contract. I returned the router to the store on 30 November and it took MTN until 6 January to cancel this contract. I therefore paid for the full month of December, where not one bit was downloaded and they further insist that I am liable for the 6 days in January as well. In other words, if you want to get out of one of these month to month contracts, you will end of paying for 5 weeks of service, where you did not receive a service. I want the contract cancelled from 1 December as that is the day I did not start to receive any service from MTN. Refund my premium for December.