Simple just disable DHCP in your SMC Wireless. Disable DHCP and then connect it to your switch/modem and it should do the trick.

You see , both routers are DHCP Server , meaning that both will issue the client with an IP. The thread starter wants to use the router as a switch , if we disable DHCP in the router [The Wireless Router , the SMC] , we're actually turning off the routing function , meaning that the router now acts as a Client [aka now it acts as an AP].