You need to give us more information about your internet service provider and which cable/DSL modem (brand and model number) they gave you. NAT Type 3 means you have a router in front of a router; your cable/DSL modem may be acting as a router and gumming things up. Apple's not likely to be able to help you in such a scenario.
You should also make sure your AirPort Extreme Base Station is up to date with the latest firmware. You can check with the newest version of the AirPort Utility.