Easy ... really
D-Link connects to the internet as it has done before. Then you set the LAN IP of the D-Link for example to 10.0.0.1 (Subnet 255.0.0.0). And you configure a DMZ in the D-Link to be 10.0.0.2. Thats all for the D-Link. Connect the !!WAN!! port of the ASUS with a !!LAN!! port of the D-Link. In ASUS webinterface in "IP Config" you set "WAN connection type" to "Static IP", the WAN IP you set to 10.0.0.2 (Subnet 255.0.0.0) and you set your LAN IP to 192.168.0.1 (Subnet 255.255.255.0) for example. And all your clients you give IPs from the 192.168.0.* IP range too (DNS Server/Default Gateway 192.168.0.1 of course). Done.