Extremely difficult. Without Recovery Mode, you may need a PC and Fastboot tools or professional service. Factory Reset Best done via Settings > System > Reset options while the tablet is still functional. System Freeze Force Restart
: When the Android robot with "No command" appears: Hold the Power button . Briefly press Volume Up once, then release both. How to Open Recovery Mode on NOKIA T21
Example: If your T21 is functional and you want to factory reset without hardware buttons, using Settings → System → Reset options → Erase all data (factory reset) will prompt and then reboot to recovery to perform the reset. Extremely difficult