@Virgo 

The problem only started when I deployed the unit in my holiday home.  It hadn't been used for about a year before that because I switched technologies at my home.

It has reset about 5 times in the last 4 days.

I'm using the original TP-Link PoE supply and my house power supply is stable and there aren't any heavy induction devices in the house to cause spikes.

While the temperature is on the high side here in France at the moment (up to 40c) the humidity is gloriously low at 35%.  

Based on other posts on this community, there does seem to be firmware versions which are more or less suseptable to this problem.  I just couldn't find one that was for a V3 unit.

You can factory reset your TP-LINK EAP110-Outdoor v1.x in the following way(s)
Through the web interface
Open a browser and navigate to the web interface of the router (see default settings below)
In the menu look for System tools and under there find the Factory defaults option
On the Factory defaults page click on the Restore button to perform the reset.
With the RESET/WPS button
Have the unit powered on
Press and hold the reset (sometimes reset/WPS) button for 10 seconds with a paperclip or other sharp object.
When the SYS led start blinking quickly release the reset button and wait for router to boot back up.

Greeting,

Rachel Gomez