landingpage for users logged-out

Hello everyone,
I’m tearing my hair out to do something that shouldn’t be impossible.

I would like to make a landing page for those not logged in. This very simple page would include a register button and a connection button, without menus and without the homepage footer. Really, just the logo and the two buttons. A kind of superpopup.

I created a child-theme from which I modified the functions.php file to include a redirect.
But it does not work.
Do you have a simple idea that would allow me to do this?
Thank you