Assuming your router is doing NAT and it supports VPN passthrough, you will need to port forward (setup done in router) the VNC traffic to the machine you want to VNC to. You will need to check to see what port your VNC software is using. I believe RealVNC is port 5900.

If you require more assistance, please let us know what VNC software you are using, what specific router you are using and the local IP address of the machine you want to VNC to.

csr