How to completely reset a Ubiquiti AP-LR.

To reset the UniFi APs, power them on, and allow them to fully boot. (In case powered by the POE adapter, make sure the cable on the LAN port of the adaptor is unplugged.)

After they’ve booted, press and hold the small reset button (located on the back of the device) for 25+ seconds.
Wait for stable Amber/White LED.(Now replug the cable into the LAN port of the adaptor)

This should reset the UniFi APs and put them back into a discoverable state again.