I had a similar issue. Xfinity Prepaid support sent me to Arris Support. I ended up trying the second solution on page 46 of the modem User Guide of "Change your security mode to “WEP” https://arris.secure.force.com/consumers/articles/User_Guide/TG862G-CT-User-Guide and I finally got both my older Samsung phone and newly delivered work Chromebook which were only connecting to WiFi to also access the internet. Samsung was at the point of recommending I send the phone in for hardware service; I am glad I found this solution.
It seems the Xfinity Prepaid modem defaults to WPAWPA2-PSK Security Mode which earlier generation devices or Chromebooks connect to the WiFi but do not pick up the internet.
6 month old dead thread now being closed.