How to grant default SMS app permission for full functionality?

Yes, you have to grant permission. And then it says set messages as your default SMS app? Yes. You have to, that is exactly what the phone is saying. It is very self-explanatory.

All you have to do is just, uh, take out the battery of the phone, put it back in, and it will, it will go back to normal.

So it allows the SMS app to work.