ZyXEL Communications 792H - Manual (Page 305)

Prestige 791R G.SHDSL Router
NAT 27-17
Now configure the IGA3 to map to our web server and mail server on the LAN.
Step 8. Enter 15 from the main menu.
Step 9. Enter 2 in Menu 15 - NAT Setup.
Step 10. Enter 1 in Menu 15.2 - NAT Server Sets and enter 1 again to see the following menu.
Configure it as shown.