Can you try pressing the WPS button on the bottom on the box? This should try and connect your device to a wifi signal. Are you using a comcast leased modem or do you own your modem?
I have the same issues. Mine worked for a month and just quit working. I called 1-800 Xfinity but they couldn't fix it. Seems these things are garbage and comcast doesn't want to fix it.
I have a ROKU and it works fine. If I can't get the issue fixed or a new box then Comcast I will just go back to just having Internet.