I've been able to successfully configure the 1711 router to forward all incoming VNC connections the computer with IP address 192.168.X.X using the following command:
ip nat inside source static tcp 192.168.X.X 5900 X.X.X.26 5900
After using SDM to configure a basic firewall, understandably, I cannot connect. I've tried several different rules but to no luck. Bear in mind I'm a newbie.