Actually, I have done that because I discovered that I can not connect to PureVPN via PPTP protocol with Voxel firmware.Welcome to the forums @mohcse.
After you flash the stock NG firmware you want to use, be sure you do a full reset to factory defaults too on the router.
In the GUI, it may seem everything is gone from Voxel/Kamoj, but some low-level bits of code may still be there to haunt you.
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