To set up CUJO with the Xfinity (Comcast) router you must use - DHCP mode. Since you can not turn off DHCP server on Xfinity router you will need to apply specific configuration.
To connect in DHCP mode you will need to plug ethernet cable in to CUJO bottom port. Open CUJO app, register and activate your CUJO. Navigate to window with manual setup instructions.
At this point:
1) Using web browser login to your router. Usually Xfinity routers address is http://10.0.0.1
2) Navigate to "Connected Devices"->"Devices". Check both "Online Devices" and "Offline Devices" lists and find a free IP address (not used by any device), for example 10.0.0.7
3) Make an empty static IP reservation by pressing "ADD DEVICE WITH RESERVED IP" button.
4) Enter "empty" for "Host Name", "00:00:00:00:00:01" for "MAC Address", your free IP (10.0.0.7 for our example) for "Reserved IP Address", "cujo" as "Comment" and hit "Save" button.
Perform this action only once!
5) Navigate to "Connection"->"Local IP Network" and change IP range to be same free IP (10.0.0.7-10.0.0.7 for our example)
6) Reboot your router and CUJO device
7) After CUJO starts smiling click "Continue" on your app
Now your CUJO will function in DHCP mode and protect all your network devices.