Click on the setup tab, and in the basic setup menu uncheck the following:

Click the Wireless tab and add a new virtual interface. Click add under the virtual interface section. 

Set the new virtual interface to unbridged. Leave the ip addresses as all 0s. (If you add any numbers here by accident, you wont be able to undo them, so just remove the interface, hit apply  and add a new one again.) Hit apply. Go back to the hotspot tab and under DHCP interface select the virtual interface you want to assign the hotspot to.  Virtual interfaces will appear in the order you create them,  for example wl0.1 will refer to the 1st virtual interface. Once you are done,  the WiFi clients will be able to connect to the splash page through a new SSID,  the LAN can connect without going through the splash page. You can hide the original SSID broadcast to ensure there’s no free access. (Wireless-Basic settings, “Wireless SSID broadcast)

You can also check out this article on dd-wrt’s website: Multiple WLANs
Alternatively a turotial sent by a customer of ours:
http://www.pennock.nl/dd-wrt/Multiple_BSSIDs.html

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