Plugin sub-menu pages recommended structure and links

You’re very close.

When you add your first submenu page you want to duplicate your initial menu page. In your second add_submenu_page you are missing the function callback for that page. Technically this parameter is optional, but it changes your url from /wp-admin/sub-menu-page to /wp-admin/admin.php?page=sub-menu-page.

Give this a shot:

add_action( 'admin_menu', 'my_menu_pages' );
function my_menu_pages() {
    add_menu_page( 'WISE Admin', 'WISE Admin', 'manage_options', 'wise-menu', 'my_menu_output', 'dashicons-tickets', 2 );
    add_submenu_page( 'wise-menu', 'General Settings', 'General', 'manage_options', 'wise-menu', 'my_menu_output' );
    add_submenu_page( 'wise-menu', 'Submenu Page', 'Submenu Page', 'manage_options', 'sub-menu-page', 'submenu_settings' );
}