ASUS RT-AC51U (RT-AC750)

→ Install OpenWrt (generic explanation)

Easiest solution for Windows users, confirmed working as of June 2019.

1. Download the ASUS Firmware Restoration Tool but don't open it yet (Recommended version)
2. Unplug your computer from the router
3. Put the router into Rescue Mode by: turning the power off, using a pin to press and hold the reset button, then turning the router back on while keeping the reset button pressed for ~5 secs until the power LED starts flashing slowly (which indicates the router has entered Rescue Mode)
4. Important (if you don't do this next step the Asus Firmware Restoration Tool will wrongly assume that the router is not in Rescue Mode and will refuse to flash it): go to the Windows Control Panel and temporarily disable ALL other network adapters except the one you will use to connect your computer to the router
5. For the single adapter you left enabled, temporarily give it the static IP 192.168.1.10 and the subnet mask 255.255.255.0
6. Connect a LAN cable between your computer (make sure to use the Ethernet port of the adapter you've just set up) and port 1 of the router (not the router's WAN port)
7. Rename openwrt-XX.XX.X-ramips-mt7620-rt-ac51u-squashfs-sysupgrade.bin to factory.trx
8. Open the Asus Firmware Restoration Tool, locate factory.trx and click upload (if Windows shows a compatibility prompt, confirm that the tool worked fine)
9. Flashing and reboot is finished when the power LED stops blinking and stays on
10. That's it! (Don't forget to re-enable any network adapters you disabled earlier)

See also Asus 'How to use Rescue Mode' FAQ

1. Download the OpenWrt firmware image
2. Unplug the power cord from the router
3. Press the reset button with a pin and keep holding it down
4. Plug the power cord in
5. Wait 10 seconds
6. Release the reset button
7. On your computer, set *this* static IP to the network adapter connected to the router (IP 192.168.1.75, Subnet 255.255.255.0, connect to the first LAN port of the router)
8. Use tftp -client to download the firmware image
   

       tftp> binary
       tftp> timeout 120
       tftp> connect
       (to) 192.168.1.1
       tftp> put openwrt-XX.XX.X-ramips-mt7620-rt-ac51u-squashfs-sysupgrade.bin

9. The power LED flashes quickly while the router is being flashed
10. Flashing is successful when the power LED is constantly lit

• After you flashed the base image, wait until the router reboots and the power LED to be constantly lit
• Connect to the router via ssh using the following command:

  ssh root@192.168.1.1

• No password is required for the first login (remember to set a strong root password for better safety)
• Follow the official documentation for the LuCI installation

• Download the sysupgrade image
• Browse to http://192.168.1.1/cgi-bin/luci/admin/system/flash (LuCI > System > Backup / Flash Firmware)
• Click Flash Image
• Upload sysupgrade image
• Confirm the upgrade
• Wait for the device to reboot (steady power LED)

Note: Depending on your firmware version, you might have to re-install LuCI and any extraneous packages after the upgrade

• Use the following commands to upgrade:

  ssh root@192.168.1.1
  cd /tmp
  wget <sysupgrade image link>
  sysupgrade <freshly downloaded image>.bin

• Wait for the device to reboot (steady power LED)

• Pull request to LEDE
• OpenWrt forum thread with infos + pictures
• GPL
  • http://dlcdnet.asus.com/pub/ASUS/wireless/RT-AC51U/GPL_RT_AC51U_30043807378.zip
  • https://www.asus.com/uk/supportonly/RT-AC51U/HelpDesk_Download/ (then Others > Others > GPL)
• It seems that 5GHz isn't supported in 18.06.2. Use snapshot for that As of Stable 19.07.1, 5GHz is working on this router
• Only power LED works, regardless of the LED configuration settings. A workaround to enable LEDs (all except 5Ghz) has been discovered on the forums LEDs are working since 19.07.6¹⁾
• Dec'21: Reports of intermittent reboots with 21.02. Link to OWrt forum
• Apr'22: Alternative name for RT-AC51U is RT-AC750. (see the original firmware image form ASUS - they compare the same).
• Serial interface is set up to be 7N1 instead of the more common 8N1

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