Switching isp.

I've had my fiber line now for almost a month and haven't had a stable connection in all that time. Every night at around 8pm the connection slows down. The Wi-Fi signal from the router that was provided with the package is just ****. I'm paying for a 20/10 mbs line but only get about 10/5 mbs. Downloading a 300mb file takes hours instead of minutes. Now who do u blame? The isp, the network (MTN) or the infrastructure (vumatel/mitchells fiber)