I do not want the router to handle IP assignment. I have something behind the router that handles the IP assignment. Basically I want the router to act as an AP. I cannot have it assigning its own IPs.
switch off the dhcp function in the web interface and - if applicable - set internet connection type to disabled, if that is also handled by the other device giving out IPs in your scenario. additionally change operating mode to router. you may want to assign wan port (see vlan) to the lan or alternatively plug the wired connection to the rest of the network into a lan port on the dd-wrt device.