I have a similar issue. I can see my devices in the Xfinitywifi Hotspot Connected Devices list, with their MAC address listed. This is how Comcast knows which devices belong to me.
I have a device I want to connect to the Xfinitywifi, but it doesn't get challenged for a username, so Comcast does not know it's mine. I get the first hour of wifi then it cuts me off. I need to whitelist the MAC address, to count as one of my 20 devices.