D-Link DIR-655 :: Local Domain Name Settings On Router

I have the DIR-655 (H/w Ver B1, F/W Ver 2.05NA) set up as router on my network (TWC/RR - modem is Motorola SURFBoard 6121 - modem only, no router or other services provided). I have configured the router sufficiently to allow all computers on the local network to access the Internet, and have configured the DHCP server on the router to provide IP addresses to the local network. I have configured the router to provide the DNS addresses for my local DNS resolvers (so that I can provide DNS resolution for an "internal" domain). Configuration specifics available on request, but everything mentioned so far is working satisfactorily, at least so far.

So, my question: The "Local Domain Name" setting on the router, as indicated in the documentation, provides a connection-specific domain name when DHCP supplies an IP address to the local system. This setting, however, is overridden if the ISP provides a domain name when it assigns an IP to the WAN connection. I have verified by experiment that this is true (no matter what I put in the Local Domain Name field - whether a valid domain, or blank - the local machines receive the domain name supplied by the ISP.I wish to modify this behavior. I wish to have the router provide the domain name specified in the Local Domain Name field *regardless* of what the ISP provides.