Hello,
It is possible, but from the WebUI. You have to login to the router via SSH and execute these commands:
uci set network.lan.ifname='eth0.1 tap0 eth0.2'
uci commit
/etc/init.d/network restart
SSH login information is:
Username: root
Password: router's admin password
To restore the WAN port back to normal, use these commands:
uci set network.lan.ifname='eth0.1 tap0'
uci commit
/etc/init.d/network restart