Boot raspberry after shutdown

Dear all,

I've just bought a DRF0494 for my Raspberry 4B and is being used only for a safe shutdown of the Raspberry. When the power source of the UPS goes down, the Raspberry shuts down itself automatically. 

That functionality works as I expected, but my problem is to boot up again the Raspberry. When I power up the ups, it starts to charge, but the raspberry does not boot. The only way I've managed to boot the raspberry was to connect the PIN 5 to GND briefly to force a reset. 

Is there a workaround to boot automatically the raspberry when I power up the ups? 

Thank you in advance for your help.