I just replaced my Xfinity gateway with an Arris SB8200 modem and Asus RT-AC66U B1 router.
After setting up the connection today, a Comcast representative provisioned my modem, and Internet works just fine when I connect my laptop directly to the modem via ethernet.
However, I have had trouble setting up the Asus router. When I launch the setup wizard, the router asks me if my IP address is static or dynamic, and a few other related questions. It tells me to contact my ISP for this information, but when I called Xfinity, the technician told me that since I'm using my own equipment, they won't be able to help me find out any of this information. I found guides online telling me that I should use the ipconfig/all command on my laptop to find out this information manually. Using that, I found out that DHCP is enabled, and assumed that my IP is dynamic. I filled in other info accordingly from the command prompt and then set up my 2.4 and 5GHz networks.
My laptop connects fine to these networks, but when I log into the router, I get an error message telling me "Your ISP's DHCP does not function properly." I can't access any web pages using the WiFi router, only a direct ethernet connection to my modem will work.
Any help with this would be very appreciated. I'm happy to give any extra info that would be useful.