If you set the DNS-323 to use DHCP client, and the router is running a DHCP server, then the DNS-323 gets its IP, subnet mask and default gateway info from the router. The DNS servers must be manually entered.
Example:
Router has a static IP of 192.168.1.1 and has a DHCP server configure for the range 192.168.1.50 to 192.168.1.99
The DNS-323 is set to use the DHCP client, it will be assigned an address (first would be 192.168.1.50) and the subnet mask would be set to 255.255.255.0 and the gateway would be automatically set to 192.168.1.1 (the IP of the router), you would have to set the DNS servers to the proper values for your ISP.
If you set the DNS-323 to use a static IP, then you must manually configure the IP, subnet mask, DNS servers and default gateway.
Example:
Router has a static IP of 192.168.1.1
DNS-323 has a staic IP of 192.168.1.2, subnet mask of 255.255.255.0, gateway of 192.168.1.1 (the IP of your router), DNS1 and DNS2 have the IP addresses of the DNS servers of your ISP.
You should not have to set any routing tables/entries using these configurations (unless you have a very complex environment - which it sounds like you do not have).