Have you connected the 24pin and the 12v?

Hello Harald, were you able to solve the problem? If so how???

When connecting a LAN cable, both lights light up or flicker normally. Otherwise, there are actually no lights on the mainboard...
At the moment I only have 8GB DDR4 RAM and Celeron G4900 LGA1151 ... and tried to get a Bios picture via DVI to HDMI... but the monitor won't budge.
Also tried to get a picture via HDMI- HDMI with RX580 8GB connected...but nothing there either...
Connecting a USB keyboard, however, causes the light on the keyboard to glow...
Power supply is EVGA 1300 G2

It can have different reasons.
Some describe the problem as having too little power in the CPU, others say it's the CPU cooler and others the RAM.
Is there any light on the mainboard?
If so, what colour and where?
It would be best if you wrote which parts you have.