Help with non-booting non-charging - resets do not help.

Samsung Galaxy Tab S7 SM-T875

This Tablet does not respond to power button at all. Connection to USB shows the empty charging symbol that indicates the better is very very flat.

Tried the various 30 second power button + up-volume combinations to no avail.

Dismantled and charged the LiPo batteries with an external charger, they both take and hold a charge - I would call them good despite having been run down to 0v.

On re-assembly the tablet behaves no different with all the same reset methods.

With the battery disconnected USB charging brings up the blank charge symbol, on connection of the battery the charge symbols disappears, so the motherboard is detecting both charging voltage and battery voltage when it's present.

Other than that there is zero response to the power button - not the slightest flicker.

I've stripped the unit down to motherboard, USB board and screen and the same thing happens, USB only gets the blank charge symbol, battery connection makes it disappear.

Otherwise the tablet is in excellent condition, no sign of liquid ingress, from what I can make out from the owner it was simply left in a drawer for too long.

If anyone has any suggestion as to how to provoke any reaction out of this thing I would appreciate it. I'm down for replacing the USB charge board or the motherboard if anyone can suggest how I might isolate which one is misbehaving.

All suggestion gratefully received!

Cheers

Pete