Hello.
Could not find a topic to this problem, so here it is.
My iconia a3-a20 tablet (2016 model, stock firmware, not much use and only a few apps installed) won't turn on anymore, no matter what

Yes, I've tried the "power+volume up/down" to no avail.
The usb connector never gave me a problem, the tablet has always been slow charging (to my knowledge, it's meant to take hours for a full charge?) but never had intermittent or faulty charging.
My take is that now the battery is depleted, but there's no way to charge it: I have tried with several charging adapters and cables (all known to work, and double checked with other devices) and the result is a sad "0" on the usb-meter key. No current is being taken from the battery, even after been plugged for a couple of days.
Unfortunately, I cannot remember if I had done a complete power-down last time I had used it, most likely I had left it "stand-by", but one thing I do remember: before it died, the Bluetooth and WiFi were not working, both stayed "WiFi is turning on...." but never worked again.
I've easily opened the case with the usual plastic tools (I can service watches, so this tablet is hardly a difficult thing!), and there's no particular evidence of problems: all the soldering looks fine, the Usb connector is solid on the board, and the battery is cool and flat (not swollen).
The battery has been unplugged from the board, then plugged in after a few minutes with no results. I do not have the scheme of the connector so cannot test the battery status (tips are welcome for this, thanks!)
Can anyone point out to a specific problem I should look for? That would be greatly appreciated, thank you!