N600 C3700 download speed issue

I have had issues for as long as I can remember with my download speeds maxing out at around 8 Mbps even though I use Speedtest.net and getting around 70 Mbps. I have xfinity internet with 300 Mbps, I am using Actiontec extender for my Desktop but have had it connected straight to the modem/router and get faster test speeds but the same download speeds. I’m tired of it and need help solving the issue.