Slow Speeds

Hello,

I just signed up for Comcast's cable package of 400 mb down, and 10 up (https://www.xfinity.com/learn/offers/details?offerId=1626100052)

I have a brand new arris SURFboard SB6190 modem, coupled with my NightHawk 1900 AC router.

When I try speedtest.net on my hardwired computer I get download speeds of 70...  When I try the wireless on my laptop I get download speeds of 60ish.   When I try speedtest on my cell phone I get 350.   

Anyone have an idea of what may be going on?  This is very strange.

Thank you,

-Robert