I just fought this issue and wanted to share a solution as I couldn't find one online.
TL;DR
After setting up Netgear WIFI Router and hooking to new Xfinity modem. I connected to the wifi and it said a sign-in was required. After clicking login it took me to the Xfinity login page.
Solution:
Change the DNS server from auto to manual and use 8.8.8.8 and 4.2.2.2.
Long Store:
My grandma moved apartments and got a new Motorola MB6800 modem. Gave all the information to Xfinity and they said they setup the modem. However, the internet was not working so she bought a new wifi router. After setting wifi router we were able to connect to the wifi but it said it required a login, which took us to the Xfinity wifi page. Internet is working on the computer wired to the router, but wifi was requiring a login. When I go to connect my cell phone to the WIFI (using router defaults), I get a pop-up saying "sign in to WI-FI network." When I click that it says, "sign in to NETGEARXX" and then it tries to redirect to Xfinity. It hangs up trying to connect to that and my phone never connects to the wifi. I have Xfinity internet, but I am not sure why they have anything to do with this netgear router and the WIFI network. Xfinity was out and spend an hour trying to fix, they provisioned the modem again, but nothing worked. The problem was that their DNS was sending us to their login page, use a public DNS avoid the issue and everything works perfectly.
