Using VPN PPTP over WAN

Hi,
For 3 days i am trying to set the firewall rules to allow VPN PPTP connection over the WAN interface. I know the VPN server is properly configure because when i uncheck "Block private networks" under Interface/WAN configuration, everything work fine. So i tired different rules and NAT to allow connection to bypass the "Block private networks" but nothing seem to work. Also i check the logs while attempting a connection and the firewall seem to block port that i made special rules to allows access! 

I tried to copy that example (http://doc.m0n0.ch/handbook/faq-webGUI-from-WAN.html) which allow access to the firewall webui from wan (of course i change the settings, port 1723 and gre proto) but no success

Did anyone know the trick to enable that feature without compromise security ?


Thanks