Simple Maintenance Mode - https://www.damiencarbery.com/2018/06/simple-maintenance-mode/

maintenance-mode-simple.php

<?php
/*
Plugin Name: Maintenance Mode (Simple)
Plugin URI: http://www.damiencarbery.com
Description: Put site into maintenance mode for regular site visitors. Deactivate plugin to disable maintenance mode.
Author: Damien Carbery
Version: 0.3
*/

// Wait until 'wp' to allow login page to work.
add_action( 'wp', 'maintenance_mode' );
function maintenance_mode() {
	if ( !is_user_logged_in() || !current_user_can( 'switch_themes' ) ) {
		wp_die( get_bloginfo( 'Name' ) . ' is undergoing maintenance.' );
	}
}