Removing the back cover usually compromises the water-resistant seal and adhesive. You will need new adhesive tape to reassemble the device properly.
Standard Fastboot ( adb reboot bootloader ) is inaccessible. The only way to revive the device is to force the Qualcomm SoC (System on Chip) into . The Test Point is the physical hardware bridge that triggers this mode. redmi 9 test point
However, the printed circuit board (PCB) contains exposed copper pads known as . These are essentially factory debugging pins. redmi 9 test point