I actually was just dealing with this today (not a RoKu) but with a new Asus laptop that uses a Ralink wireless card. It turns out the Arris Gateway has a lot of problems connecting devices. Calling Comcast tech support (ones who only work with Xfinity Arris Gateway) they said they have been having major issues with the new Xbox360 Slim, Ralink wireless chipsets, and the new Broadcomm wireless chipsets.
The only solution was to bridge the gateway (turn off DHCP and the WiFi) and use a separate router. Basically the gateway turns into a normal modem and then you connect whatever router you want. We did this and everything connected fine thereafter.
You can google search this and find a lot of people having the same problem. It has to be turned off by Comcast because you cannot do it your self from the gateway config page.