Based on what is written at the wiki, I installed 14.07 Barrier Braker with the factory.bin as my initial flash.
I updated the startup script with the line 'sleep 2 && /etc/init.d/network restart' before exit.
I set both country codes to United States for 2.4 and 5 ghz.

So here is the bug for 14.07 Barrier Braker I get.
I cannot start my 2.4Ghz Generic MAC80211 802.11bgn (radio0) without enabling my 5Ghz radio first.
No matter how I set up the adapter, it wont work unless both 2.4 and 5 ghz are on at the same time.

So I decided to update to the latest snapshot since that apparently fixed it.
I originally tried m00nman's builds but they didn't support the luci-app-qos package so I went to the standard snapshot, which hasn't been updated since march (probably a build bug).
My wireless has the same issue except now it drops if under high load.
Netflix causes it to go out after about 30-40 minutes and the router then has to be rebooted.

Does anyone have any luck with this device currently?
Any advise for me?
I really need QoS and stock firmware doesn't cut it.
Padavan doesn't support QoS either.

This may help someone to help:
Ralink RT3662
https://wikidevi.com/wiki/ASUS_RT-N56U

I also needed QoS on this router, I flashed moonman's build (he commended I do it via ASUS firmware utility), after failing with Padavan GUI, Stock GUI, and TFTP. I'm running the OpenWrt Chaos Calmer r45496 / LuCI (git-15.107.38351-ec09e99)  build from him and it's working fantastic, QoS and everything. No more buffering for me on Netflix or Hulu when using my Roku 3 Box. Been running stable for a few days already, only time I went down is when I start tinkering with things

Also, 2.4G and 5G worked right out the box without needing to fiddle with scripts , even though I was prepared to mess around..

(Last edited by rensorek on 28 Apr 2015, 03:33)

Thanks for the reply!
I will attempt to flash that build with the Asus utility right now.
I will report back how it went!

Edit:
Just flashed openwrt-r45496-ramips-rt3883-rt-n56u-squashfs-factory.bin with no luck.
My unit doesn't start the LuCI and I can't even telnet / ssh into the unit.
I can't even ping 192.168.1.1 after setting my IP to static 192.168.1.11.
Going to try his openwrt-r45496-ramips-rt3883-rt-n56u-squashfs-sysupgrade.bin and report back.

Edit2:
So I'm up and running, but I have the same bug where I can't turn on 2.4ghz without enabling 5ghz first. I'll test and see if it is stable.

(Last edited by deese.john on 28 Apr 2015, 05:31)

My RT-N56U also works fine with the official BB14.07 no fiddling with scripts needed at all and i can enable or disable both 2.4 and 5 wifi's independently. Could there be different versions of the RT-N56U?

according to https://wikidevi.com/wiki/ASUS_RT-N56U

there are 2 versions: v1 with Ralink and B1 with Mediatek

I can confirm the FCC ID is MSQ-RTN56U.
Padavan's firmare always worked fine but I really need the QoS.
I seriously can't stream one netflix instance without my home's internet becoming useless.

EDIT:
So my router wouldn't allow anyone to access the internet, had to reboot the unit.
I'm using the following packages (all luci-app- versions): ddns, upnp, and qos.

(Last edited by deese.john on 29 Apr 2015, 00:52)

Just checked and I have the same FCC ID. Try flashing openwrt-r45496 sysupgrade via OpenWRT.

(Last edited by rensorek on 29 Apr 2015, 01:38)

I'm running moonman's r45496 build. The wireless bug with needing to start 5g to get 2.4g to work is still there. I've tried the normal snapshot r45496 with no better luck. Going to stick with moonman's build for now considering he seems to be specifically supporting the device.

All I can say is try starting fresh. Install full STOCK asus firmware. Then use the asus firmware utility to flash openwrt-r45496-ramips-rt3883-rt-n56u-squashfs-factory. I just did a reset on my version and erased everything then tried enabling ONLY 2.4GHz without touching 5GHz. It worked just fine.

When you say the Asus firmware utility, are you referring to the web administration page, "Firmware Restoration" or something different?

I attempted to flash the factory.bin with the restoration utility before and was unable to connect to the internet. I couldn't ping, telnet or ssh the unit.

EDIT:
So I've tried everything...
Trial 1) Asus firmware utility flash Stock -> Asus firmware utility flash openwrt-r45496-ramips-rt3883-rt-n56u-squashfs-factory.bin
Results in unit with "nothing turned on" requiring either serial cable or reflash.
Can't SSH or Telnet, nor web interface with the unit.

Trial 2) Asus firmware utility flash Stock -> Web Interface flash openwrt-r45496-ramips-rt3883-rt-n56u-squashfs-factory.bin
Results in unit with "nothing turned on" requiring either serial cable or reflash.
Can't SSH or Telnet, nor web interface with the unit.

Trial 3) Asus firmware utility flash Stock -> Web Interface flash openwrt-ramips-rt3883-rt-n56u-squashfs-factory.bin
Now I can access the web interface.
The web interface allows me to "enable" 2.4ghz but there is no signal on my wifi analyzer and no device sees the SSID.
I then enable the 5ghz and the 2.4ghz now shows in both the analyzer and devices can no connect.
I can then disable the 5ghz and 2.4ghz will stay up and work fine for a few hours.
The router will then lock up requiring a reboot.
This results in the same 2.4ghz bug, requiring me to start the 5ghz first to get it to work.
The router will eventually lock back up.
The line "sleep 2 && /etc/init.d/network restart" (without quotes) before "exit 0" does not change the behavior of this in anyway.
I have the country code selected as US but I've also tried 00 - World.

Trial 4) Trial 3 - > Web Interface openwrt-r45496-ramips-rt3883-rt-n56u-squashfs-sysupgrade.bin
Results in the same issues as trial 3.

I am going back to Padavan's firmware and I guess I'll just have to suffer without QoS for now.
Stock firmware drops internet connection for me, so it isn't an option.
This unit has been awful and dealing with Asus is impossible.
If anyone has any idea how to fix this, I will be forever grateful.

(Last edited by deese.john on 29 Apr 2015, 23:24)

On the Asustek website there is this Utility if you search for your router:

ASUS RT-N56U Firmware Restoration version 2.0.0.0

Use this to flash OpenWRT. Worked for me since neither Padavan, Stock and TFTP were able to flash OpenWRT properly. I would end up with only a power light on and no way to connect to anything..

I've been using that utility. No luck.

Did you install with the web interface?

I decided to attempt Chaos Calmer 15.05 since it just went stable.
I flashed the factory.bin image and my router would not boot.
After using the Asus firmware utility, I reset to Barrier Breaker and used the SysUpgrade image.
The wifi bug (can't enable 2.4GHz without 5GHz) is still there!
I'm going insane, about to smash this router with a hammer and buy a router that has better support.

I have this Wifi bug too (5 before 2.4). 15.05

Nowadays using this script

uci set wireless.@wifi-device[1].disabled=0
uci set wireless.@wifi-device[0].disabled=0
uci set wireless.@wifi-iface[1].disabled='0'
uci set wireless.@wifi-iface[0].disabled='0'
uci commit wireless; sleep 5; wifi down; sleep 5; wifi up; sleep 30; wifi;

but anybody knows better way to start Wifi at startup after disabled Wifi5?

Does your 2.4 stay stable using this script?
Even after activating / deactivating 5 for 2.4, my 2.4 would eventually drop.

Cant you just leev 2.4 and 5 ghz radios on and give them both the same SSID if turning the 5 GHZ radio off makes the 2.4 unstable?