Very slow speeds

I was having connectivity issues so I purchased a new modem, the Arris SB8200. i already called to activate it. My speed isn't anywhere near where it should be considering that I'm on the Premier plan. Using the Speedtest.net site, I found  that I'm getting approximately 17Mbps down and 9.56Mbps up. I tried 3 different browsers and they all said about the same thing.

I ran that same test on a Verizon mobile phone and it said 49.38Mbps down. How is it that I get a faster connection using mobile than I do with Cox where I'm directly connected to a router (not using WiFi)? I should just tether my phone instead of using Cox.