Then it seems a matter of something simple not being done, maybe disabling DHCP and setting static IPs?
As far as increasing range, that's a matter of increasing output from the router (also configurable through DD-WRT) and/or changing TX/RX antennaes to high gain models or even signal amplifiers.
Once the WDS is successfully set up, range is a matter of manipulating your environment, antennas and transmit power (from amplifiers, computers, etc)
Ryan