Double charges and unable to cancel contract

I have had endless problems with Telkom since trying to cancel my contract. The 24 months expired, I tried to cancel online with no luck, tried the call center and was put through to other departments and kept on being disconnected. Finally I went to a Telkom shop. The friendly lady helped me log into the online portal and cancel the contract. She said to expect a call confirming the cancellation. Said call never came. That same day I get a bill from Telkom via email for 4 months back dated charges. All 4 of these periods were paid on time. These are double charges. I also keep receiving monthly invoices for this contract that was cancelled and the device (LTE router) is switched off and not in use. I phoned in a couple of times losing hope time after time as I kept being cut off after holding for insane amounts of time, transferred from one person to another, no one able to help me or cutting me off. Last week I phoned 5 times and after 45 minutes on the phone finally got someone on the phone who was trying to genuinely help me. She said that as long as there are outstanding charges on the account (whether valid or not) the contract cannot be cancelled and that she would escalate the dispute and that I have to wait to hear from Telkom. I am almost certain I would be a fool to wait. What do I do now? It’s the fourth month that this is dragging on & I’d really like to clear the false charges and close the account but it seems almost impossible?