UPS does not work when iMac is in stop mode

Hello everybody,

I'm actually living in a house where there are frequent blackouts. Therefore I bought an UPS for my iMac (2017) 2,3Ghz 21 inches, SSD hardware. The UPS works perfectly when the iMac is turned on, but when it is on sleep mode and there is a blackout, I hear the sound of the UPS that is in action but the iMac is shutted-down. So I try to turn it on but it doesn't work. Then the only way to turn the iMac on is to shut down the UPS and then press the power button of the iMac.

As I said, everything works well when the iMac is operative, but not when it is in the sleep mode.

I'm wondering why this happens....

Maybe there is some option that I can set up on the iMac?

Thank you

Following the feature of the UPS

Epyc Neon

output capacity 650 VA/360W

220-240V battery mode

Output frequency 50Hz/60Hz +-1 (battery mode)

Transfer time 4ms

battery type 1X12V/5Ah