Yes that is possible. But you have to separate the wireless and LAN into two networks.

You need to ssh to the router and edit the configuration files directly.

In /etc/config/network, add a lan1 network for wifi

config interface lan1

option proto dhcp

 

In /etc/config/wireless, change network for wifi to lan1

config wifi-iface

option network ‘lan1’

In /etc/config/dhcp, enable dhcp for lan1

In /etc/config/firewall, create a new zone for lan1, and enable data forwarding from lan1 to WAN.

Sorry there is some configurations and I cannot give you exact detail before I try.