@njk01 , To configure PPTP, you have to forward the TCP port 1723 (control data of the PPTP connection) on your modem.
The second part of communication is the connection with GRE (Generic Routing Encapsulation). This is where the PPP packets are tunnelled. What you have to forward here I can’t tell you.
Do you have your own router at home? If so, you can set the modem to bridge mode and then adjust everything on your router.