MTN does want to cancell my LTE account... after cancelling and returning the modem in the shop, they are still billing my account

MTN does want to cancell my LTE account after letting us jumping through hoops!!! After cancelling and returning the modem in the shop and calling theurbuseless call centre, they are still billing my account. My MTN LTE account was canceled and paid up last month, but MTN is still trying to bill me.