My M20 had same problem too. I think below link is useful. I think Miravison has some problems.
Please read Trackieman’s review and don’t choose “Restore default settings”.
https://www.banggood.com/Original-Box-Teclast-M20-MT6797-X20-Deca-Core-4G-RAM-64G-Android-8_0-10_1-Inch-Tablet-p-1333450.html?
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