I have a wifi router...

I have a wifi router with them and last week Wednesday I cancel it to take a cheaper one, so today I phoned to ensure that the router is cancelled because I received a new router which is cheaper today, so now they tell me that month end I will have to pay a full amount for the router that I want to cancel, because the payment will be for the month of March whereas on the 25th March they have debited my account.