Home Internet nightmare

I recently took a month to month home Internet from mtn online which came with a free to use router, due to their fair usage policy the subscription didnt work for me, i had to cancel the contract which i returned their free to use router and all its cables, i have since received a invoice which states i owe a total of R5498 which i dont know what this amount is for, i sent them countless of emails to which no one from the billing department has came back with feedback regarding the cancellation of the debit order and the amount of R5000. Mtn is a ******* i regret ever using their services