Which "instructions" did you follow? Is there a link?
I'm trying to do the same thing and seem to have found an interesting "feature"...
I've been using my generic instructions:
http://www.coastal-computers-networks.com/Wireless%20Router%20as%20a%20Simple%20Switch%20and%20Access%20Poiint.pdf
This basically says:
Turn off DHCP
Don't plug into the WAN port at all.
Plug the LAN into one of the LAN ports.
What I found was this:
Normally, following these instructions, it doesn't matter what IP address is assigned to the router - except for convenience.
The idea is that there is a processor in there with an IP address assigned AND that the "switch" in there (i.e. which the LAN ports connect to) doesn't care about IP addresses just like most switches (and most switches in routers).
What I found was that there was no communication between the switch ports (i.e. LAN ports) unless the router IP address was in the same subnet.
Very strange but, indeed, convenient when you have to reach the router control panel from within the LAN subnet.