screen, connector or motherboard issues?

i have a phone that i’m told had a screen replacement but the it stopped working.

i tried that screen on a working phone and it worked fine.
I tried a known working screen on the phone and no display.
when i plug in the charging cord, i can feel the taptic buzz and the audible ding.
when i connected the phone to 3uTools I was able to see that the phone boots, even installed the latest IOS and activated it. so it’s pretty clear the motherboard works.
it appears that even the digitizer works took as i held the power button for 10 seconds then swiped right where the “slide to power off” button appears and 3uTools indicated that there was no longer a phone connected.

i realize this is not enough for a proper diagnosis, but what are your thoughts?
it doesn’t look like it’s anything in the display or display cables.
it seems to read the digitizer.
the connector looks to be in good shape, but that’s hard to say.
if it’s something on the motherboard, what are some of the things it could be? and what’s the difficulty of repairing?

Thanks