I've spent the last 3 days trying to fix this and am at a loss. I woke up Saturday morning to find we had no internet and it had gone out sometime around 1 a.m. I rebooted the modem and router. Nothing.
With my laptop connected to the Arris Modem via ethernet, I had internet most of the time. When I connected to the router via ethernet, I would have internet sometimes but kept getting an error that said "Your DNS server may not be available."
I got another router and tried that. Same error.
Tonight a tech came out to check the outside lines and said everything was fine. I replaced the coax cable (which goes straight into the wall, no splitters, etc.) and got a new modem. I can access the internet again on ethernet to the modem, but when I try to use the router, I have no WiFi. I haven't bothered trying the ethernet from the router.
Here are the stats:
I don't know if this will solve your problem, but I recently got a firmware update for XFi that broke my wifi and I have a temporary workaround. This firmware upgrade was supposed to increase my internet speed from 400/10 to 500/15.
tl;dr: Use the XFi mobile app to use different SSIDs for your 2.4 GHz and your 5GHz networks.
Xfinity rolled out a firmware upgrade to increase performance, and also to encourage the use of their mesh network (see XFi Pods). In this upgrade, they started enforcing that the 2.4GHz and 5GHz networks use the same SSID. Using the 10.0.0.1 interface, you can no longer separate the SSIDs, but you still can make this change from the XFi app.
The new firmware upgrade however, causes problems when the SSIDs are the same - I am getting dropped connections all over the place making the internet anything but usable. Once I split the SSIDs, it works great on each SSID individually. Prior to the firmware upgrade there was no problem when they were on the same SSID.
To make the change in the app, click on the network name (SSID) on the main page, click Edit Wifi, check the "Use different names and passwords for 2.4 GHz and 5 GHz WiFi", and then put in a different name for your 5GHz network. Many devices still only support 2.4GHz, so I would recommend leaving that one as the original.