Very slow wifi connection at night

We have had a very, very slow connection for a while now. But the past 2 weeks, we can't do anything at night. Only one device can be on the wifi. Currently, we are paying for a 30 meg line, then at night, we don't even get a 1 meg line or any wifi at all. I have contacted the support group on rain itself. Each time I get a message, we will attend to your problem. Nothing. Last night I contacted the call centre and the lady said someone would get back to me nothing. If we have a slow connection because of congestion then please let me know, then I can look for another service provider. Why must I pay the monthly fee, and can only use one device at night, and then you still need to watch whatever in low quality because the wifi is slow. We have reset the router numerous times, so the router isn’t the problem. When I do a speed test in the morning, we get a 30 meg. Can someone contact me so this can be sorted out if not, then I will have to make another plan, but I can’t pay for a service and can’t even enjoy the service.