Charging for a service I cannot use

In March 2021 they changed my internet speed from 100/50 to 200/100 but the router that they originally installed cannot process at 200/100. The new price for 100/50 is R989 but they kept billing me R1299 for the 200/100 line, knowing that I cannot use that speed. This change was done without expressly notifying me and giving me the options to stay on the 100/50 line at a deceased price or get a new router that can process 200/100 and stay on the current fee. This I only found out 9 months later when I complained that my line speed is down to 40/10. When I asked that they refund me the R310 that I have been paying for a service that I cannot use they refused and blamed it on Openserve. Clearly they will do anything to rip their clients off by making changes that are not authorised or even requested. Extremely rude accounts department people with zero understanding, there only answer is "we cannot refund you".