Cancelling internet link with no coverage

At the end of August 2019 the agents of Telkom, Mobile Talk, sold me an 20GB internet link after I've given them my address and they checked for coverage. When I received it there was no coverage and I reported it to them. They then checked again and confirmed no coverage, cancelled it and arrange for them to collect their stuff, but they continued to deduct from my account. After more complaints they returned about 95% of all their deductions from my account in Desember, but also continued deducting. With more complaints their argument was that these deductions was for add-ons that I never cancelled. My return argument was that they can't add to something that they sold to me under false pretences that I would have coverage, that they already have returned deductions for these and that they cancelled it because of their fault, not me.