Port 1723 is the port used by PPTP, which is NOT an IPSec VPN, so IPSec passthru will not help. I am curious why you are trying to port forward to a PPTP server, if you purchased a VPN router, that is designed to be used as a VPN endpoint for your client to connect to?-----

[SOLVED] PPTP VPN setup on RRAS server not working Apr 27, 2020 Windows VPN client connect on different port - Server Fault Is it possible to have the Windows PPTP VPN client (on XP in this instance) connect on a port other than 1723? If so, I can simply port forward another port to the second server. I've done a fair bit of Googling over the last few days and have only found others asking the same question but no answers. I have of course tried to add a port number port 1723 blocked by sbs 2011 - social.technet.microsoft.com Dec 29, 2011

Jan 24, 2020

Set up VPN Server - Synology DSM Help PPTP VPN service is built-in on some routers, the port 1723 might be occupied. To ensure VPN Server works properly, you might need to disable the built-in PPTP VPN service through the router's management interface to have the PPTP of VPN Server work. Configuring a VPN server in Windows 10 - Where am I going

vpn port 1723 | Tech Support Guy

8 rows What Is PPTP: Point-to-Point Tunneling Protocol Jul 09, 2020 How to Open Port 1723 for VPN | Tech Support Guy