Ridiculous billing!!!

MTN is the worst. I went to a gateway branch to cancel the contract and I owe nothing. I then get an invoice billing me R1650.00 I then called the billing team at their call centre and they say they are charging me for a router - why the hell are they charging me for a router that was retuned to their branch? They can go pull a CCTV because that router was taken back as it was required before cancellation commences. I am irate!! And I’ll not pay that stupid money - this is day light *******.