One short update here, may be a info that could go into the wiki also.
I use the switched +5 V power ot the ATX PSU to power the Duet and for switching on the duet I need to
keep the PS_ON lead from the ATX PSU grounded manually for ~3 seconds, after the blue LED near the wifi module
flashes a bit brighter and goes off.
Than the duet PS_ON pin goes to GND and I can release my mechanical push button.
Note : I decided to go this way, because I do not want all the electronics to consume power without any purpose.
I have (currently) no plan/need to switch the printer on remotely without being there to watch the print start.