To allow traffic through your router, you should open up the port used by PPTP. It’s likely you can access your router’s configuration page at 192.168.1.1 or 10.1.10.1.
Now, assign a private IP address to your device (so this private IP address will not change over time) and allow TCP traffic on port 1723 for that device on that private IP.
- IP Protocol: TCP (or, choose ‘any’ if possible to also include UDP traffic)
- Port number: 1723