thanks for your help

If you dont have child theme then create using this plugin easily https://www.ads-software.com/plugins/one-click-child-theme/.

function md_maintenance_mode() {
if ( ! current_user_can( 'edit_themes' ) || ! is_user_logged_in() ) {
$static_page_URL = 'https://www.examplecom'; // Must Change it
wp_redirect( $static_page_URL );
exit;
}
}
add_action( 'get_header', 'md_maintenance_mode' );

Thanks
Mohammad