I've a AP900 works fine on the poweradaptor.
But if I remove the power it restarts and stays in a reset loop.
Connecting the power and the issue is solved.

Even tested with factory settings.

I use firmware version 1.2.0.

Sounds to me as thought the POE power is not adequate enough for the AP to run?
Could be a cable fault?
I have heard of some switches supplying 24v instead of 48v, think it is a new standard?
I have an AP900, 902 and several 910C all working fine on 48v POE switches.

WHS - Is it a proper 802.3af (or at) PoE switch/injector ?

My AP behave similarly when connected to TL-SG108PE - reset loop
Fw 1.2.1.1

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Edit Piaf in 2018-04-28

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some AP models have a 5GHz wifi module defect that results in reboots, after turning off the 5 GHz module, the reboots stop

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Edit say...
Maybe connected topic
https://forum.draytek.co.uk/viewtopic.php?f=19&t=21299&p=87644&hilit=POE#p87479