Because the MX9 is a generic box, the motherboard inside can change week by week. Flashing the wrong firmware (even if it is for an "MX9") can permanently your device.
If you'd like, I can help you if you tell me: The board version number (e.g., R329Q V8.0) The chipset name (Rockchip or Allwinner) firmware tv box mx9 4k android 712 verified