I'm being billed for a service I don't receive

Telkom continues to bill me for a data contract which I signed up for and terminated the next day because there was no coverage at my home even though their agent had assured me that my area is covered. I asked repeatedly if the wifi device would work because I had a previous mobile wifi contract where connection suddenly became weak and intermittent towards the end of the contract. I was assured that the new device would work because it was on the lte network and not 3g like my old one. I took the contract on the 20th of April 2016 and cancelled it on the 21 of April 2016 because it was not connecting to the internet when I brought it home. I signed up and cancelled the contract at the Forest Hill store in Centurion and returned the device and sim card to the same place on the 21st of April 2016. I have the signed cancellation form to prove it. I have called their call centre so many times and have been escalated to call centre managers but Telkom kept deducting monthly premiums for this device and data. Early in 2017 I decided to block their debit order from my bank after realising that they were not capable of resolving the matter on their own. I have been charged for almost a year and up until this day I get sms notifications and email statements stating that I have an account that is in arrears. Please help recover my money and stop this madness as these losers are ruining my credit record. My contact number is ********** 924.