Download speed drastically dropped

My fibre download speed has tremendously dropped. I'm on a 20/10 Vuma Reach subscription, all has been well until recently when I started to experience streaming issues and also general connectivity issues. When I check the speed using the Speetest app it shows my download speed is below 5Mbps. I notice my internet speed is also not constant, it ranges between 1Mbps and 6Mbps for most of the time, in the evenings and night it drops to even zero.

Webafrica did the usual, asked me to turn off my router and switch it back on after 10mins as they refresh the router ports. It seemed to have worked, but only for a few minutes.

There is no answer as to why my download speed has dropped, and this is so frustrating as I'm not getting the service I signed up for. This is ridiculous!!! I need my internet download speed restored or I will have to cancel as Vumatel and Webafrica fail to keep their part of contract obligation.