Wrong billing

I ordered a router last year. The agent who sold said it have network coverage where I lived. When it arrived it never worked, went telkom offices they said the router is working on town. Not it the area I stayed in hence it displays no service. I cancelled the agreement immediately because I couldn't pay for something I'm not using. I got reference for cancellation. But even today Telkom is busy deducting my money for this even though I cancelled it long time ago. They still bill me and I have to stop them with the bank. Now my credit record is negatively affected by this. If they don't sort this out I'm willing to take them to Oubudsman for financial services. When I call they still send me from pillor to post