Wise Old Owl
Due to overwhelming popularity of Intel, no compatible AM4 motherboard is available for testing with the 3 local repair guys I have contacted.Make some inferences after testing if another PC with another board (preferably with your PSU) turns on or not at the same outlets, again testing both with and without UPS/Inverter. Ask the local repair guy to carry his board+cpu+RAM & his power cable to your place and his PSU to test both ways (using yours as well as his PSU).
Using live wire tester & a DMM have you confirmed if the power cable is actually carrying enough voltage at its other end?
Due to the Corona situation home visits by repair techs is an issue. Still I managed to get one guy and check. But he did not bring a whole PC, only PSU.
Electrical testing was all done by electrician. I have a multimeter bought sometime back but due to my being busy professionally there is not enough time and patience to learn how to operate it and do the testing. It is a pending project for the time being.
Presently this is my only desktop at home. I thought the motherboard and any other part I buy now for testing may later on be used to build a backup system. But the damn confusion AMD has created with AM4 and CPU/chipset generations is causing me anxiety. I don't want to be stuck with 2 motherboards which support only older generation processors later on but newer boards may not support my gen. 1 CPU