Screen completely dark, external monitor works

I got this iMac used, the previous owner told me the connector to display on motherboard had broken off. I had the connector replaced by a professional repair shop, but the screen still won’t turn on. Not just backlight, the screen is completely off, not even showing in system information. (I tried to shine a flashlight into it, nothing)

If I hook it up to an external monitor, it works as the only display device, so I think the video card is fine..

Now what could be the problem? assuming the connector and display cable are working correctly (I tested every pin with multimeter)

LCD itself? Display power board?

I’d appreciate some expert opinions, thanks!