Who is your ISP? It may be they are throttling your service if you use excessive amounts of data. We all know they are not supposed to do that. Have you tried running a speed test from a neutral site like? speedtest.net
I have also heard rumors if you don’t rent the equipment from the ISP they won’t guarantee the speed. If there are any other devices connected to the router, directly or by wi-fi at the same time it can kill your bandwidth.