Shut down the modem and the AirPort. Boot up the modem and allow for all lights to appear correctly. Then insert a paperclip into the reset hole on the back of the AirPort, plug it into power, and hold it until the status light blinks rapidly. Then release and reconfigure in AirPort Utility. You need the AirPort to be in DHCP & NAT mode for this to work. The SB6121 is a simple modem that I use myself at home and it simply passes one IP through to the router. The router then has the job of creating a local area network and assigning IPs.