Have you tried to make a restore of Windows using a USB-Stick? Probably it will not work because if you cannot enter into the BIOS, the USB-Stick will also not be loaded. But at least you could try it.
The right key to enter the BIOS in the Cube Mix Plus is the “Del” key. Try it again pressing the key repeatedly from the moment that you power on the tablet.
Last hint: press the “Tab” key when the screen is in the chinese leters. You should see the BIOS information and maybe you could see there a message telling you something important, like “press F6 to continue” or whatever.
Let me know if you find the solution or some new relevant info in order to try to help you.