Apple's [http://www.apple.com/airportexpress/ Airport Express] and [http://www.apple.com/airportextreme/ Airport Extreme] have some settings that make it hard to use with other networking devices, including your Amahi server.
This trick also applies to the AT&T Uverse routers that do not enable to control DHCP by their users.
There is no option to disable the DHCP in the Airport admin tool, so what you need to do is trick it into being "more pasive".