Factory reset maybe the best choice if you cannot open the machine. Step 1. Turn the tablet off by using power off option if the tablet turns on or Pressing and holding down the Vol. UP key and the Power Button until the screen goes black. Step 2. Press and hold both volume buttons, while holding Vol. UP & Vol. DOWN you will press and hold the power button for exactly 1 second (count on your head or say it out loud -Mississippi one-) and release the power button BUT KEEP HOLDING THE VOLUME KEYS DOWN. Step 3. You will hold down the Volume keys until you see the screen turn on and the logo show up. (WHEN YOU SEE YOUR TABLET LOGO INMEDIATELY RELEASE THE VOLUME BUTTONS)

Probably the wifi chipset is faulty. Recommend buying a new one or sending for repair. Apparently it is not a software problem.

If you ensure your wifi network function normally, there should be some problem with the wifi component on the chipset of your phone. Unless you really love your phone, otherwise, replacement is recommended.

It is hard to justify by using model number and ram amount only. Yet a factory reset may work.

Maybe 1) the display is damaged 2) the display functions normally, but cannot receive information from the processor, some of the connecting parts must have problems. As you have mentioned, it gradually become back, so it seems to be 2). The phone functions correctly except for the display. I recommended sending it back for repairing. It will be very risky as you cannot determine which connector(s) are faulty, and it did not seem to be a software problem.