How to set up macOS Server’s VPN service | Macworld
Unfortunately, it is not possible to change OS X's built-in PPTP client's port. However, you can use a third-party client such as OpenVPN. OpenVPN has many more options and will allow you to specify the port with which to connect to the server. Good luck and happy tunneling! Setup a VPN server with macOS Sierra server 10.12 Jun 24, 2020 Introducing port forwarding on the router
Turn an old mac into your own Virtual Private Network (VPN
Nov 16, 2017
First off, what protocol of VPN are you using? OS X Server offers both L2TP (over IPSEC) and PPTP protocols, both of which use different ports. For L2TP you need ports 500 (UDP), 1701 (UDP), and 4500 (UDP). Forward these ports to the same ports internally. For PPTP, it would be ports 500, 1723 (TCP), and 4500, also forward the same internally.
Port forwarding is a technique used to enable incoming internet connections to reach your device when using a VPN. It is necessary because most VPNs use an NAT firewall to stop users falling victim to malicious incoming connections. This is a useful security feature, but it is unfortunately unable to distinguish between incoming connections you don’t want and those that you do.