Activation of modem/router (without app)

Not sure if this method works anymore (YMMV). Try calling 1-855-652-3446. That is their self-install device activation line. Have the RF / CM / HFC MAC address and the serial number of it handy when you call. Good luck ! Please let me know.

Good afternoon @user_gyznpl, I hope you are having a great Friday! Thanks for reaching out to us here on Xfinity Forums. As @EG says, you can call in to our customer service department for assistance with setting up the modem, or if you would like we can help with the activation also. Could you please send our team a direct message with your full name and full address?  
To send a direct message:
Click "Sign In" if necessary
• Click the "Direct Message chat" icon
• Click the "New message" (pencil and paper) icon
• Type "Xfinity Support" in the "To:" line and select "Xfinity Support" from the drop-down list which appears. The "Xfinity Support" graphic replaces the "To:" line
• Type your message in the text area near the bottom of the window, please add your full name and address
• Press Enter to send it