Can’t activate new modem/router in app

Update:  spent nearly an hour on a chat with Xfinity. They appeared much more interested in selling me new service than resolving my issue with the new modem. The chat disconnected with no resolution. Does anyone know if it is an Arris issue or an Xfinity issue?

Try calling 1-855-652-3446. That is their self-install device activation line.

If that doesn't work for you, you can call 1-800-comcast and tell a rep that you would like to have a new modem provisioned for service. Have the RF / CM / HFC MAC address and the serial number of it handy when you call.

You've reached the right team for help activating your new Arris SBG8300 modem, @user_849fc3. Thank you for trying the self-service options to activate! We very much appreciate the assistance, and for you sharing that information, @EG. The SBG8300 is on the list of compatible devices. Let's take a closer look, and work together to get your new modem online. Could you please send a direct chat message with your full name and complete service address to “Xfinity Support”? To do so, click on the chat icon located at the top right of this forum's page.
 
Here are the detailed steps to direct message us:
• Click "Sign In" if necessary
• Click the "Direct Message” icon (upper right corner of this page)
• Click the "New message" (pencil and paper) icon
• Type "Xfinity Support" in the to line and select "Xfinity Support" from the drop-down list
• Type your message in the text area near the bottom of the window
• Press Enter to send your message

@user_849fc3 

Did either one of those other methods work for you ? Please reply right here on this public help forum.