The efforts that you made to explain the Internet to your grandmother were in vain. They were fruitless. This is the meaning of 'beat' or 'bang' your head against a brick wall' (= to keep trying to do or achieve something that will never be successful).

Hey Safaa!!! OK, this expression means: To waste time & efforts trying to do something impossible to be done. Let's put it in that line You wrote: "I've been wasting my time, wasting efforts trying to explain the internet to my grandmother, because She will NEVER understand it." ^__^

That is just a figure of speech... It does not necessary mean the person is banging their head on the wall. What she is saying is that she is so confused, that she feels like banging her hand on the wall, because her grandma does not understand the internet.