MTN Canival mall does not want to cancel my contract, they debit while I returnes their router.

I canceled a month to month contract with MTN in January this year. Returned the router however, they are still debiting that money for router that is not in my possession, I wet to the MNT at Carnival mall where I returned it. However, they are also giving me a run around no one seem to want to assist. Everytime I go there I'm being told the manager who has to assist is not at work. It's almost 6 months fighting one and the same thing. Can someone please advise what must I do for this matter to be fixed.

I called their head office, they tell me to go to the store, no one wants to assist and resolve this matter.

Regards