|To use the SMC as an AP you will have to have it plugged in to a LAN port on the Siemens router.|
You will go from LAN port on the SMC to LAN port on the Siemens.
You will want to assign an IP on the LAN side of the SMC that is outside the DHCP scope (on the Siemens) but within the same subnet. The subnet mask needs to be the same and the gateway IP should point at the Siemens router. Then you'll want to setup the wireless. Use the exact same SSID on the SMC that you're using on the Siemens but make sure you put it on a different channel to avoid overlap and interference. So have one set on channel 1 and the other on 11.
The following is an example of network settings to give you an idea what I'm talking about:
Seimens Router LAN settings:
DHCP scope: 192.168.0.100 to 192.168.0.199
SMC router LAN settings:
Default Gateway: 192.168.0.1
You will want DHCP disabled on the SMC as once everything is setup and working properly clients connecting to it will get their DHCP from the Siemens router's DHCP.