Here is how I would do it:
Unplug all of the client PCs.
Connect modem to wan port on Dlink router.
Connect Dlink Lan port to Siemens Wan port.
Power cycle the Dlink, then the Siemens.
Log into the Dlink and see what IP is assigned to Siemens. Probably 192.168.0.2
Log into Siemens and configure the Wan to use a static IP. Enter the IP found in the previous step.
For default gateway, use the IP of the Dlink router. Probably 192.168.0.1
For DNS servers, use your ISP's name server addresses.
Go to the Lan DHCP section of the Siemens and set it to use 192.168.1.1 through 192.168.1.255 for assigning IPs.
Reconnect client PCs and switches/hubs. Use crossover cables or uplink ports between the routers and the switches/hubs.
I'm fairly certain that is how mine is set up and it works fine