how to factory reset Mikrotik hAP ac lite

Process to factory reset mikrotik: Disconnect power → press button → connect power → wait until "usr" LED starts blinking → release button → Wait until router boots up.

Do not rely on measuring seconds. Blinking is what matters. Common mistake is to hold it too short (does not reset config) or too long (enters into "CAPs" mode which is advanced setting for large networks).

Full description of LED blinking states:
1) After powerup, it blinks once, for fraction of second
2) Then turns solid light for approximately 4 seconds. If you release it during this stage, it will load backup bootloader and start with existing config.
3) Then it starts blinking for approximately 5 seconds. If you release it during this blinking stage, it should factory reset.
4) Then it turns solid light for approximately 5 seconds. If you release it during this solid light stage, it should reset config and switch into CAPS mode
5) Then it turns the light off. If you release button now, it will go to netinstall mode