I bought my parents an AirPort Extreme base station, because the cheap wireless router they use doesn’t broadcast a signal strong enough to reach the back bedroom. I’ve connected it to their cable modem, run AirPort Utility on my MacBook and asked it to configure the base station for a DHCP connection, but it always comes up with a self-assigned IP address. When I reconnect the old wireless router, it works perfectly. What am I doing wrong?
You need to reset the modem, but how you go about it depends on the modem. In some cases you can simply unplug it, let it sit for a few minutes, plug its output into the base station’s WAN port, power on the modem, power on the base station and then run AirPort Utility. With luck, the base station will pick up a usable IP address and be on its way.
However, some cable modems come with a backup battery that maintains their settings even when unplugged. If your parents’ cable modem has such a battery, there’s a good chance it also has a small reset button on the back. Turn it around and look for it. If and when you find it, give it a firm push (you’ll probably need a paper clip or pen-point, as these buttons are often recessed). This should allow the base station to pull in a working address.
If not, internet providers are often capable of resetting modems from their end. If nothing else works, call tech support and ask them to reset the modem.