Actually this guy didn't mention the Firmware version of the Unify AP LR.
If his firmware was 3.4.* or less than this method is correct and I believe that he was using old Unifi AP LR not V2.
I am pretty sure that this method described above will not work with Unifi AP LR v2.
If someone know how to flash new version of UAP LR or UAP AC LR, and can describe the method, it will be really really useful. I a, ready to pay money to this person!
I have Unifi AP Outdoor and I can confirm success with downgrade ubiquiti's firmware from 3.9.something to 3.3.20. I did it with Ubnt's Controller v5.6.29 on Debian 9 for uploading both images (I renamed lede-17.01.4-ar71xx-generic-ubnt-unifi-outdoor-squashfs-factory.bin to BZ.ar7240.v3.3.20.4019.160401.1459.bin althoug not sure if necessary).