Xiaomi Mi 9 not turning on

It doesn't turn on at all, there are no vibrations when trying to turn it on but the led on the front of the screen does turn on sometimes when I try to turn it on but than it just goes out after a few seconds.

I took it apart and everything seemed fine, I tried reconnecting everything but that didn't work and after that I tried cleaning all of the connectors with rubbing alcohol but that also didn't work.

I also tried turning the phone on with a charger.

What could be the fault?