After you switch on the wifi extender, power light will blink few times then other LEDs will blink. Power LED will be on for few seconds and again the whole sequence will repeat. I suffered this almost for a week. It would suddenly connect with wifi router but after you switch off at night and put it on again, whole story starts again. So I went thru community messages and could only read the same complaint and TP link same standard comments. 

So I connected it to ethernet cable and powered it on. Same issue repeated there too. I left it like that for few hours and kept on checking on Tether mobile App. Moment it got connected and all LEDs were ON except the wifi one, first thing I did was firmware update. Connect to your device go system, go to firmware update and hit update. It's working now. 

Key is you have to get the extender up with ethernet or wifi router originally connected with it.