Hi all.
Just to share my happiness.
After more than 1 year trying, I manage to use TP-Link MR3420 v2 as my UniFi router.
The step by step was similar with this blog. Credit to the author:
http://myjowenunifi.blogspot.com/And below is the final config:
Switch:
port WAN is equal to port CPU in the web interface
port 1 is equal to port 2 in the web interface
port 2 is equal to port 3 in the web interface
port 3 is equal to port 4 in the web interface
port 4 is equal to port 1 in the web interface (for HyppTV)

LAN:

WAN:

IPTV:

Network config:
CODE
config interface 'loopback'
option ifname 'lo'
option proto 'static'
option ipaddr '127.0.0.1'
option netmask '255.0.0.0'
config globals 'globals'
option ula_prefix 'fd7f:3234:32ba::/48'
config interface 'lan'
option type 'bridge'
option proto 'static'
option ipaddr '192.168.1.1'
option netmask '255.255.255.0'
option ip6assign '60'
option _orig_ifname 'eth1 wlan0'
option _orig_bridge 'true'
option ifname 'eth0 eth1'
config interface 'wan'
option _orig_ifname 'eth0'
option _orig_bridge 'false'
option proto 'pppoe'
option username 'username@unifi'
option password 'password'
option ifname 'eth0.500'
config interface 'wan6'
option ifname '@wan'
option proto 'dhcpv6'
config switch
option name 'switch0'
option reset '1'
option enable_vlan '1'
config switch_vlan
option device 'switch0'
option vlan '1'
option ports '0t 2t 3t 4t'
option vid '500'
config switch_vlan
option device 'switch0'
option vlan '2'
option ports '0t 1'
option vid '600'
config interface 'IPTV'
option type 'bridge'
option proto 'none'
option ifname 'eth0.600 eth1.600'
Has been successful tested on:
OpenWRT-AA 12.09-rc1
OpenWRT-BB ROOter Huntsman
OpenWRT-BB ROOter Multiweb