The privacy screen protector alone can't do the job, although we have them in different quality, but what I usually do is also change my phone to dark mode and reduce the phone's brightness to a minimum, which only me looking directly at the phone can see what's going on there freely.
Have you used one before? Probably it is fake or inferior. I have told my brother before that how come I do not see what is on his phone when he is using it? He told me he used privacy screen protector for it. He said he bought it for that reason. You will not be able to see anything on the phone from the side view if you get the good one.
Buy it from a trusted seller. Make sure you explain what you want to the seller, and tell the seller to put it on your phone for you and test it to know if it is good or not, but make sure you told the seller that this is what you want and let him give you assurance that it is before he opens the screen protector.
Yes, I have used one, and I'm currently using it on my phones. One thing that I have observed is that, from a side view, everything indeed will be blank and won't be seen by someone who is trying to look into your screen, but anyone who is standing behind you, like at your back can see that's why I recommend a reduced screen brightness, which also protects our eyes.