It has an option to enable one of the Ethernet Ports as the WAN which is what I have done and connected an ethernet cable from eth1 to the Wan port.
Make this a network port, not a wan. Have all ports as lan, no dhcp, no gateway or whatever enabled.
In simple words: it shall behave as a switch with a wireless access point. All the work will be done by lmce (dhcp, routing, firewall, etc.....)