MWEB debited my account without providing the service

I had an ADSL line with Mweb and in 2016 I signed an application form for Mweb fibre. After waiting about 18 months Mweb installed the fibre and charged me R1099.00 p/m. However Mweb did not cancel my ADSL line and continued charging me an additional R1036,63 p/m. When I contacted Mweb they refused to refund me with the ADSL line charges although they have not been providing this service. When I installed the fibre they knew that I was terminating the ADSL but they continued debiting my account for the ADSL. I find this totally unacceptable.