Motorola SB5100 Cable Modem will not connect

I have a Motorola SB5100 cable modem that I plan to use with Comcast. Unfortunately, Comcast cannot activate it. I have a second SB5100 and it works fine on the same connection.
The previous owner used it with Insight Communications and it worked well.

All the LED’s work properly. The “on line” LED blinks confirming that it’s not connected.

Any and all recommendations and comments will be appreciated.

Thank you