Wordpress Plugin: add_action function seems not to be called

Question

I lightened up one of my plugins to the maximum (which does several things), but in this specific case it only has to deal with a simple "maintenance mode".

I was testing the plugin on a fresh Wordpress installation (5.9.3), but the function (TestMaintenance) is never called.

<?php
/*
* Plugin Name: Test
* Description: Test
* Version: 1.0
* Author:Test
* Author URI: Test
* License: GPL3
* Text Domain: my-utility-test
*/

if (!defined('ABSPATH')) {
    die('Direct access not allowed!');
}

class MyCustomPlugin {
    
    public static $instance;
  
    private function __construct() {

        add_action('get_header', [$this, 'TestMaintenance']);
        
        register_activation_hook(__FILE__, [$this, 'activation']);
        register_uninstall_hook(__FILE__, 'uninstall');

    }
      
    public static function getInstance() {
        if (!self::$instance instanceof self) {
            self::$instance = new self();
        }

        return self::$instance;
    }

    public function activation() {
        return true;
    }

    public static function uninstall() {
        $options_list = [];
        foreach ($options_list as $option) delete_option($option);
        return true;
    }
    
     
    public function TestMaintenance() {
        wp_die("<h1>Maintenance</h1>", get_bloginfo('name'), ['response' => 200]);
    }

}

MyCustomPlugin::getInstance();