Hi, say you haven't entertained the idea of totally removing the headache and selling or otherwise getting rid of one of those horrid Zyxel things?
Anyway... please find below your user and setup manual, (You mentioned TWO types? So i got manual;s for both?) below, also there is a website that should automatically "Sort your problem"....
Also it may pay to upgrade your Firmware too, there may have been "Fixes" that will help.
http://www.belkin.com/support/article/?lid=en&pid=F5D8235-4&aid=14407&scid=942
www.devicemanuals.com/.../ZyXEL/ZyXEL-P-660HW-D1-3-40-User-Guide-TkRjNU9EUT.html -
http://portforward.com/
Also remember to do ALL settings changes as Administrator, at all times...Make sure you actually apply, and set the settings. Always save the changes in the Router, then reboot router, and then go and actually check, thew changes made have been actually changed, sometimes, see, it may take a few goes?
Use that portfowarding.com, you should be able to, using them, sort your issue.
You need to also check, that your ISP doesn't "Block" certain "Ports" as they often do. There is little point trying to forward an un-forwardable port.
The manual seems to explain it pretty well, just work through step by step, repeating these steps over and over sometimes, check, check and recheck, all settings and entries..