Unable to activate with Xfinity - sb8200

SB8200 is just a cable modem without any wifi and it’s just a plug and play device. So to test the modem status please connect any device to the modem ethernet port and check for the IVP4 address and also try accessing modem GUI on direct connection. Also you can check whether the modem is online on the Xfinity app or if that shows offline you can do self activation to activate the modem manually.

http://arris.force.com/consumers/articles/General_FAQs/Xfinity-Cable-Modem-Activation