For advanced users who have unlocked the bootloader on the NOKIA T21, Fastboot offers another route.
Wait 5 seconds to ensure the device is fully off. How to Open Recovery Mode on NOKIA T21
Recovery mode lets you troubleshoot and restore your Nokia T21 when the tablet won’t boot, is stuck in a boot loop, or needs a factory reset. Use these steps carefully—factory reset from recovery will erase local data. Back up anything important before you begin. For advanced users who have unlocked the bootloader
: Press and hold the Volume Up and Power buttons simultaneously. is stuck in a boot loop