Have you checked that there no component missing near the display connection socket ? unfortunately I have the same issue and i think is due to some capacitor broke during the flat removal :( i bought a new display, and both old and new make no life signal
Be careful prying up the connector and apply force only from the side as shown in the picture cause in the longest side there are capacitors that can be broke doing the lever ( happened to me Repair missing capacitor near display connection port )
