do you have more than 1 device connected to the sb8200? 
It has more than 1 ethernet port but xfinity only supports 1 device connected to it (unless you pay for a business class line). 

If you had more than 1 thing connected, power the modem and the router off for a couple minutes and disconnect the coax from the modem. Wait a bit. 

Then connect the coax back to it and power it on. Give it a couple minutes to fully boot 

Then connect the router to port 1 on the modem and power the router on. 

give it a couple minutes and attempt internet setup on the router. 

Thank you - I followed the steps but no luck. One item to note is that my modem does plug into a a large router that feeds up to 15 different remote expanders throughout the house. There are other components after the modem feeds internet into the house. For example, a surgeX is making noise. 

What router is connected to it? 

Does the modem work if you connect a computer to it?

You’ll have to follow the procedure above but using the computer. 

An araknis 

I have not tried with direct line to computer - will do so now