Being harassed by Telkom

I took a contract wifi router with them only to find out that there was no telkom signal where I stayed. I contacted them and they told me to send the router back, indeed they sent a courier to pick it up. All these happened back in July 2019 but they kept on taking money on my account every month since then until april these year. Then they started calling me and telling me that my account will be referred and they did that though I have reference numbers indicating cancelation, they have now black listed me, when I call them they say there's no such account with them. I don't know what to do anymore.