PPTP - Enable GRE protocol using xfinity mobile app

To get PPTP to work, I understand you need to run two steps in the xfinity mobile app:

1. Port forward 1723/tcp (I figured out how to do that).
2. Enable the GRE protocol

How do you enable the GRE protocol in the xfinity mobile app?
Past forum posts show how to do this in the web app, but the web app is no longer an option.

Thanks.

PS: I will use PPTP at my own risk (for historical purposes and educational use)
and I understand OpenVPN is the recommended choice for most users.
However I am still interested in using PPTP for educational and retro computing purposes.