I'm a couple of years late but in case anyone finds this thread from a search, I hope this might help:
I got the 'self-assigned IP' problem after reversing the order of two Airports - base vs extension.
At first, I tried tips that helped others - trashing various prefs, re-setting the base station, manually entering random DHCP client numbers, deleting networks, keychains, etc - but no luck.
I then tried the following:
- turned Wi-Fi off on all Macs
- unplugged the 'extension' Airport
- reset the base station Airport (paper clip/flashing orange light routine)
- reconfigured the network with Airport Utility (using the same network name and password as before)
- turned Wifi back on and - bingo - both - Macs connected to Wifi again
- I then restarted and reconnected the 'extension' Airport.
I can't say which step(s) fixed it but my guess is that it may have been a combo of simplifying the network to just the base station, then resetting that, and then rebuilding the network.
Steven