I suppose your LAN is connected to the ethernet interface ETH00 and your WiFi LAN to the ETH01. I do not know if your zeroshell box is also a wireless access point or just you connect the ETH01 to external AP, but it is same.
If the wired LAN and the wireless LAN must share the same subnet and hence the same default gateway and the same dhcp server, you need to create the bridge BRIDGE00(ETH00,ETH01) and assign it only one IP included in the subnet. This IP will be the default gateway for wi-fi and wired clients.
You will be able to use this IP to administer zeroshell by using either ssh or the web interface. The WAN interface (I do not know if you use a PPPoE, UMTS/HSDPA or ethernet interface, but is the same) must be included in the NAT interface list from [Router][NAT] so all the source IP of the outgoing packets will be masqueraded with the public IP of the router.