Submenu Page under Settings in Admin Panel

I didn’t test this but i’m pretty sure you just need to remove your code from the admin_menu() function declaration.

Like this:

add_action('admin_menu', 'register_my_custom_submenu_page');

function register_my_custom_submenu_page() {
    add_submenu_page(
        'tools.php',
        'Submenu Page',
        'My Custom Submenu Page',
        'manage_options',
        'my-custom-submenu-page',
        'my_custom_submenu_page_content' );
}

function my_custom_submenu_page_content() {
    ?>
        <div class="wrap">
        <h2>Page Title</h2>
        </div>
    <?php
}