After what seemed like endless search over short period of time, and much wasted time on phone calls to support reps who were not familiar with telnet.. I finally stumbled across this article below from howtoforge

Big thanks goes out to sugree and cpbotha on this one and all the guys at howtoforge that helped google index their site so I could find this article. 🙂

1. Telnet to the router and enter administrative password
2. Go to menu 24 and then 8
3. Run command "ip nat loopback on"
4. Type "exit" and then 99 to quit from the management screen

For a little background history, this was originally a double NAT scenario. If you have another router and the ZyXel, my recommendation would be to disable DHCP on your second router, enable it on the ZyXel and connect the second router to the ZyXel via LAN port instead of the second routers WAN port. This will utilize the second router as a bridge and maintain a single subnet instead of two separate subnets. All port forwarding can then be done from the ZyXel.

References
http://www.howforge.com/4-steps-to-turn-on-nat-loopback-in-zyxel-router