WiFi very slow - CAT-5 at 720Mbps; 3 feet away

Things were just fine with my Arris SBG-6782, no issues; but then i got an email from Comcast (the first time ever) stating that I need to upgrade my modem.  As i was paying for 300Mbps, I found that I could get 600Mbps for $10 lesss per month; so I upgraded.  I purchesed a Arris SBG-8300, and now with a much faster and bertter modem my WiFi speed is only at 20 -45 Mbps, but the wired is at 720Mbps.  I've called comcast several time's for more thany 6 hours over the phone and had them out at my location two time's and they can't find anything wrong.  When I caled Arris they went through the system and stated several times that it was Comcst that has not opened several channles and that was causing the slow speeds for WiFi; Comcast is stating (over the phone) it's the Arris, the Comcast tech's that came out both stated that it was comcast not the Arris.

I then got a replacement Arris and am now having the same issue.  

Why can Comcast fix this issue, what can be done?

Please Help

Eric