Billing problems

I took out a WiFi contract (2 devices for 1 price) on 27th December 2019 only to get home and not have coverage, I took the devices back the following day and requested cancellation which they confirmed to be done. However I'm being debited R510 & R410 every month, when I call I'm assured it will not happen again. I have been experiencing this problem for 6 months. The most recent case numbers are: ***8 & ***8. May I be assisted please.