Cable modems usually need to be "blanked" before you can connect a new router or computer to it.
Power down the cable modem.
Wait 30 minutes (go and have a cup of tea or something).
Connect the cable modem to the WAN port of the 802.11n Airport Extreme Base Station (nAEBS).
Switch on the cable modem.
Switch on the nAEBS.
Wait five minutes.
Switch on the Mac.
Use the Airport Utility to configure the AEBS.
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