Can't access Windows 7 PPTP VPN from external network

[AMD 32]

I've set up a PPTP VPN through the built in Windows 7 features but I can't access it from outside the network, only internally. This is with my Firewall completely disabled and DMZ enabled to my computer's internal IP address. I don't plan to keep it that way of course, but I can't see why it won't connect from outside the network even when all the ports should supposedly be open. The error is just "The connection could not be established to the PPP server." This is from an iPhone and MacBook Pro running OS X Lion. Any ideas?