All the tempered glasses made for m21 or m31 are smaller in length and lower portion of the phone in unguarded and not smooth to operate afterwards. I think you should purchase m31s tempered glass, which can fit better for your phone. But I can not give you guarantee that it will work. You can always return it, if you purchase it from Amazon.
I'd recommend a film screen protector fromGadgetshieldz which covers all edge to edge and is installed with liquid to reject all the bubbles. The protector has a self-healing feature that heals most of the scratches that occur on it so it looks new as always. I'll share a link for you, check it out. https://www.gadgetshieldz.com/samsung-galaxy-m21-2021-screen-protector-guards.html
