https://help.ubnt.com/hc/en-us/articles/204910124-UniFi-TFTP-soft-recovery-for-bricked-access-point
UniFi - TFTP soft recovery for bricked access point
Overview
Readers will learn how to properly recover a UAP device using the TFTP method.
This method should help recover a "bricked" UniFi AP. This method will NOT void your warranty whereas opening the chassis for serial TTL WILL void your warranty. Most soft brick issues will be resolved via this method.
If you are going to attempt serial TTL recovery it is TFTP recovery as well. Feel free to contact support if you have questions about the serial TTL method. There is very little else that can be done. Usually when these methods don't work properly it indicates a hardware issue with the device that cannot be resolved via software.
Notes:
Since controller 3.2.5/4.5.2, UAP-AC and UAP-AC-Outdoor support TFTP recovery, so just use the firmware included with the controller.
UAP-Pro and UAP-Outdoor+ require a special image for TFTP recovery. It is 16MB and not included with the controller. Please contact support if you need this binary.
Steps
Prior to beginning the TFTP recovery, identify the correct firmware by going into your controller (also see notes below steps). You can find the correct fw path on your controller for the device you need to revive by using:
The chart found on the bottom of the page HERE, or
The following file on the controller: [unifi_base_dir]/dl/firmware/bundles.json
For eg., I have a Debian 7 x64 controller, ver. 3.1.3.2187, and I needed to revive a Unifi AP std, so my path was: /usr/lib/unifi/dl/firmware/BZ2/3.1.3.2187/firmware.bin
Then, set a static IP on your PC's NIC from 192.168.1.0/24 range, but not 192.168.1.20 (this is the Unifi AP default TFTP IP).
Follow the steps to unbrick your UAP:
Set the IP on your PC to be able to access the bricked unit and prepare the firmware file.
On your PC, open your TFTP client and locate the firmware.bin file so you can start it later. Do not initiate the transfer.
Using the Windows integrated TFTP client the command to prepare is:
"tftp -i 192.168.1.20 PUT [path to file]\firmware.bin"
Unplug the bricked unit.
Plug the LAN connection of the PoE injector directly to your PC's NIC.
Keep the UniFi AP's reset button depressed and plug in network/PoE in the unit.
Keep the reset button depressed until you see the light cycling relatively fast through amber/green/off colors (~14 seconds from power on) -> Release it. Now the device is in TFTP transfer mode.
Engage the TFTP push command and wait. The device will write the firmware and it will reboot.
If you wait too long to start the TFTP transfer, the push will not work as the device will stall. Please redo from step 3.
At this point your device should be recovered and ready to go. If it is still not functioning as expected then you will need to send for RMA (if under warranty).
- UBNT-MikeD