First, you can use both wireless signals. Set them to the same network name, same encryption mode, same encryption key, but different channels. (Don't pick channels close together, like use 1 and 6 or 1 and 11.) Clients will automatically choose the strongest signal.
You must disable the DHCP server on the DLink router. Then connect one of the DLink's LAN ports to one of the Siemens LAN ports. Do not connect the WAN/Internet port on the DLink to anything.