Make sure that you power off the cable modem completely by pulling the power cable from the back of the device or the wall AC socket.
Pull the battery from the modem if it is easily accessible
Hard Reset the AirPort Extreme by holding in the reset button for 9-10 seconds and then releasing. Allow a minute to restart
Power off the entire network....all devices
Wait at least 30 minutes. Some ISPs take even longer for the equipment back at the cable company to reset and issue a new fresh IP address for the modem
After the wait period, reinstall the battery in the modem it was previously pulled
Power up the modem and let it run for at least 5 minutes by itself. 10 is better.
Make sure that the Ethernet cable from the modem is connected to the WAN "O" port on the AirPort Extreme
Power up the AirPort Extreme and let it run 2-3 minutes
Power up your computer and reconfigure the AirPort Extreme
Check for proper operation
Power up other network devices one at a time about a minute apart