Hi @bakhtawarzahid,
How long does the tablet stay on for when working on the battery only after it has been fully charged with the tablet off?
If only a short while then the battery may be failing and the charger can't do both i.e. charge the battery and operate the tablet at the same time.
Try replacing the battery and check if it now works OK.
Here's a [https://www.youtube.com/watch?v=rKL0Y29aRiU|video] that shows what to do.
Search for ''SWD-WT-N8'' to find battery suppliers that suit you best.