Airprint unable to find HP Laserjet 1536dnf via Zyxel P-660HN-F1z

Airprint is very disappointed. I recently baught *HP Laserjet 1536dnf* *which Apple and HP says it is Airprint enabled* only to find that it doesn't work. And I think both Apple and HP may blame it on Zyxel but the one who is frustrating is a customer like me. If anyone can help me ... please.

_Here's my configurarition._
Device: Macbook pro, HP Pavilion, iPhone 3GS (iOS4.3), iPad 1 (iOS4.3)
Printer: HP LaserJet 1536dnf (latest firmware 20101125)
Router: ZyXel P-660HN-F1z (latest firmware Version: 3.70(BJI.4) | 03/22/2010)
Network Setting: All are in the same network 192.168.1.x

_Here's my problems._
Printer works fine with notebooks (wired) but not iPhone and iPad (wireless). I have tried many settings and after rebooting my router and printer I will occasionally be able to print from either iPhone or iPad but never from both. Most of the time, iPhone and iPad show "unable to find printer"

_My tests were._
- Update all device to latest firmware
- Reseting router and printer
- Deactivate my firewall
- Setting my printer to a fixed IP
- Setting log in my router (and I don't understand it!)
- etc
None of them works.

I guess I may have to do something like openning port for Banjour or some apple services on my router but I really don't know what to do or how to do it.

I'd appreciate if anyone can help or *share the router setting* that works for your airprint.