Phone is dead. What can be the reason?

We have a Galaxy Tab 10 that has stopped working. It seems like the tablet doesn’t turn on.

Things I’ve tried:

1. Bought a brand new charger - didn’t change anything.
2. Opened up the case, removed the screws and disconnected the connectors to the motherboard. Couldn’t find anything corroded or broken. The only weird thing was mic module is full of black goo. It doesn’t look like glue, maybe melted plastic. I disconnected it from the board.
3. I cleaned the connector module from dirt and checked the electrical connection between connector and the motherboard using a multimeter and there are no breaks.
4. Cleaned and reassembled.

Still it doesn’t work. It is a 10+ year tablet so it makes sense it just generally dies, but just out of interest and challenge I would love to know what you think can be wrong and how to troubleshoot deeper than the troubleshoot guide here.

Ideas would be welcome, and specifically one question:

Does the tablet have an indication when it powers up beside display, like for example my phone both turns on the screen and vibrates. Does the tablet also vibrate (knowing this will help me narrow down the problem it might be having).

Thanks :)