Xfinity wifi migration to Google mesh wifi

You need to put the Comcast rented gateway device in to bridge mode. Here's how;

https://www.xfinity.com/support/articles/wireless-gateway-enable-disable-bridge-mode  

Then the Google main router gets connected to port number 1 on the Comcast gateway.

Follow the instructions for the Google Mesh System for the balance of the configuration.

@user_b5e610 Thank you for reaching out via our Xfinity Forums. I see that @EG was able to provide you with the steps needed to set up your Google Mesh Network. He is a valued asset to the support we provide here, and we truly appreciate the assistance he so gracious provides. Please let us know if you have any other questions, and we would be happy to help! Happy Holidays!