At this point you should perform a complete power recycle of your networking components.
The following should be performed as a minimum:
- Power-down the modem, AirPort base station, and computer(s).
- Disconnect the AirPort base station from the Internet broadband modem.
- While all of the devices are powered-down, perform a "factory default" reset on the base station. This will get it back to its "out-of-the-box" configuration and make setting it up much easier, especially if you use the "Assist me" process within the AirPort Utility.
- After the base station resets, go ahead and power it back down.
- Reconnect the AirPort base station to the Internet broadband modem. For the Extreme and Time Capsule, be sure to connect the cable to the base station's WAN (circle-of-dots) port.
- Power-up the modem; wait at least 10-15 minutes to allow it adequate time to initialize.
- Power-up the AirPort base station; wait at least 5-10 minutes. Note: The AirPort's LED status light may continue to flash amber after it has initialized. That is because, there may be some additional configuration items necessary, like setting up wireless security, before the overall setup is completed to get a solid green LED status light.
- Power-up your computer(s).
After a factory default reset, the AirPort base station will broadcast an unsecured wireless network with a Network Name (SSID) of something like: Apple Network NNNNNN. Network clients, connected to the base station either by wire or wireless, should now be able to access the Internet through the ISP's modem. Once Internet connectivity has been verified, you can use the AirPort Utility to configure the base station for wireless security and any other desired options.