Comcast has at least a half dozen different models out there. You didn't say which model, so no-one can answer your question. Remember too, some models pre-date the WPS feature.
Have a look at the Comcast page and see which model matches what you have at home. Then go to the makers web site and download the instructions.
Answers & Comments
The SMG Modem Wi-of router has the button right on the top front edge of the device. Give that a try.
Comcast has at least a half dozen different models out there. You didn't say which model, so no-one can answer your question. Remember too, some models pre-date the WPS feature.
Have a look at the Comcast page and see which model matches what you have at home. Then go to the makers web site and download the instructions.