Still being charged on a cancelled contract

I took out a LTE router contract in March. I received the package on Wednesday the 16 March. I couldn't get the SIM connected. I spent 2 days calling call centres - holding the line at 1 hour intervals. I finally was told the SIM automatically gets activated after 48 hours. Friday comes, I use the internet for 1.5 days - the whole months usage is depleted!!! I phone to ask why. They say you have to switch off the router if you don't want it to run background tasks. Hmmm, no one informed me. Anyway I phone the call centre to cancel this contract. They say I'm within the 7 day cooling off period and that I won't be charged. I go to the Telkom store in Northgate (22 March) to cancel this contract. I fill out a form, ask the agent if I'll be charged. She says "no" as I'm in the cooling off period. Two days ago - 3 May I get an account for R 776 even getting billed for the whole of May. I "query my bill" via the online chat service. I go through the whole story with Liezaan and she says I need to go to a Telkom store to sort this matter out. I then followed her advice and went to the Telkom store in Fourways Crossing. I was told by an agent that they don't handle cancellations in store at all. I have struggled with this contract from the beginning. Told false stories. Run around, and wasted my time calling, emailing etc. I WILL NOT SPEND ANOTHER MINUTE ON THIS MATTER - AND WILL NOT PAY THIS BILL. IN MY EYES I HAVE DONE EVERYTHING TO TRY SORT THIS MATTER OUT.