I am guessing your Motorola Modem is an SBG900 it is the only Motorola I can find that has the same default address as your D-Link G-54 192.168.0.1 If it is the above mentioned Modem you do not need the D-Link the Modem has a built in router in it. As far as being able to or how to access your D-Link I have no Idea of how you are going to do that.
The only way I can think of is to change the default address of one or the other. But I do not know how to do that. The second link is a guide to port forwarding through 2 routers if you need it. Best I can do is say good luck for now.
I will do some more investigation latter if I have time.
http://broadband.motorola.com/consum...Data_Sheet.pdf http://www.portforward.com/help/doub...forwarding.htm