Being charged for a MTN router that was likely ****** by one of MTN employees

I cancelled my contract with MTN in August and I returned their router as per the agreement I had with them. They have been billing me for the router ever since and I have reported the issue and how much of an inconvenience it is to me but they have not been able to assist me. They have beeN investigating God knows what but they continue to debit me for the router. It seems as though one of their employees stole the router and now Im paying for it. I have provided proof to indicate that the router was returned! I’m so livid with MTN. Please be cautious with them.