I set up a DIR-825 in standard router mode on a business network. It is not at the edge, it is inside the network to provide wireless access.
The inside of the router is a 10.0.0.0 network and the network (WAN) side is a 192 network.
Then I created a route to 192.168.0.0 on the DIR-825. This allows wireless users (with 10.x IP's) on the 2.4 and 5.2 bands to access the fileserver and Exchange server on the 192. business network.
Unfortunately, it also allows users on the Guest Zone network to do the same. So there is no isolation there.
What possible (good) reason could there be for the router to apply routing table entries to the Guest Zone?