force login and redirect to custom login page

If you’re willing to try another plugin, I suggest using Force Login. It’s a simple lightweight plugin that will require all visitors to login.

Once the plugin is installed and activated, add the following code to your functions.php file to change your site’s login URL to point to your custom login page:

function my_login_page( $login_url, $redirect ) {
    return site_url( '/haendler-login/?redirect_to=' . $redirect );
}
add_filter( 'login_url', 'my_login_page', 10, 2 );