The Siemens Speedstream DSL modems are actually more than just modems, and can cause significant conflict with routers, namely IP conflicts causing the inability of the router to connect to the modem.
The solution is detailed fairly well for a netgear router here and the same general method can be used for dlink routers as well, the options are just in different places. The modem is actually aware of the IP space and creates an internal network with 192.168.0.* space that most routers use by default. It gives itself the IP address 192.168.0.1 which is identical to the default IP of DLink routers, so they will never communicate unless one is changed. If the computer is connected directly to the modem, going to 192.168.0.1 will get the modem configuration page, whereas if you are connected to the router you will get the router config page.
Basically the solution is to configure the modem to set it to always attempt to connect and get the DNS server IP addresses, then set the router to use a different IP space like 192.168.2.* for itself and the internal network. The router must also be set to connect to the internet directly using the static ip address 192.168.1.64, gateway 192.168.0.1 and subnet mask 255.255.0.0. I'm guessing you can use any ip address in the 192.168.1.* space to assign to the modem, but I used 192.168.1.64 and it worked ok.