I just acquired a fantastic IBM Aptiva 100% complete (incl. original CDs), it's a P166/32MB/3.2GB model
It turns on and works fine, however the power supply is now 30 years old, it's very light (usually not a good sign of quality), extremely dirty inside and the FAN is very noisy.

Now, noise and dirt can be fixed with cleaning and new FAN, but the unit is closed with rivets (first time I see this) so I'm not sure how should I open it without some drastic solution like drilling the rivets?

Anyways, as I already had a bad experience with the power supply exploding on another Aptiva (desktop variant), I would really feel better with a modern ATX PS + AT adapter so I bought this:

And I also verified that voltages do match with the original connectors, as IBM for some reason swapped orange and yellow. All good, even -5V is present.
However, when I plug everything and turn on the ATX power supply, this immediately gives power to the connectors, and the FAN on the Aptiva mainboard starts spinning, also some controllers get slightly warm, but the system does not POST
I dod some research and the board/CPU might be in a reset loop, waiting for a signal to power ON, however I'm not sure how can I reproduce this with the adapter setup?

The mainboard has this two connectors

One is connected to the power button switch, P9 is connected directly to the power supply. It has 3 cables, black-white-brown. Shorting black and white allows to power on the power supply if it's not connected to the mainboard (had to learn this to measure the voltages) so I suspect it has something to do with the power on sequence of the Aptiva, but can't figure out how.
I stupidly tried to short the corresponding pins to black and white on the mainboard while the system was in reset loop, but did not work.

I asked ChatGPT, he is talking about tension difference at startup that the board uses as a signal to turn on, but I'm sure how should I reproduce this or if it's even tue or an hallucination.

I'm definitely out of my comfort zone here, I'd really appreciate if someone could help me!