TCL 75S423 black screen but LEDs are okay. Which board to replace?

Common situation: dark screen, tv is on, sound is on, flashlight test show parts of image.

So, based on internet research, I found that the LED strips are okay (Got a $20 LED tester, unplugged each strip's connector and tested. Each of the 12 strips lights up at 38.8V).

The power + LED driver board LED connector measures zero or close to zero voltage. (2 outputs measure 0.7V, the rest are at zero)

I also measured the red connector, going between the power board and the main board (I think). All 3 24v pins read 24v, the BL_ON, when TV is powered, read 3.2V, and the PS_ON read 2.5V.

The most amazing thing was, that while measuring the BL_ON with the multimeter, all of a sudden all the backlight LEDs turned on! This 'fixed' the problem even after a power cycle, and I was hopeful. Well, after a few minutes the magic was gone and the TV is dark again. No magic power here...

Does that mean that the main board is the faulty one? How do I further figure it out?

TIA