You don't configure the IP address inside the router but inside the laptop/desktop PC instead which is going to be used to configure the WG602. (Reset the router to its original settings) My WG602V4 used to connecting to a DG804G
Go to the laptop/desktop PC's network adapter properties (you didn't mention which OS) , fix the IP address to 192.168.0.210 and then connect the WG602 to it using an ethernet cable. Power up the WG602 and make sure that the LAN light is lit.
I believe you've the WG602 manual already, here is how to configure IP address in a Vista PC click here (Win7 is similar ,XP is simpler)