You have to configure YOUR router to accept TCP port 5500 (or what ever port you have chosen with the VNC software) in from the outside. 5500 is the default port. This is called NAPT or Network Address Port Translation. (similar to NAT but uses ports as well as ip addresses instead of just ip addresses).
Depending on the router, you may have to allow his to allow traffic out on that port.
here is a link with some helpful solutions that may get you going in the right direction.