Get the page IDs of a Particular Menu item’s submenu

You can try the following function:

function wpse_290320_get_page_ids_from_menu( $menu_id, $submenu_id )
{
    $menus = wp_get_nav_menu_items( $menu_id );
    $pages = array();

    foreach( $menus as $menu )
    {
        if( $submenu_id == $menu->menu_item_parent && 'page' == $menu->object )
        {
            $pages[] = $menu->object_id;
        }
    }

    return $pages;
}

$menu_id = 26; 
$submenu_id = 3298;

$pages = wpse_290320_get_page_ids_from_menu( $menu_id, $submenu_id );

You can find the menu ID if you select a menu from Dashboard > Appearances > Menus, then in the URL. For example, if the URL is http://single.dev/wp-admin/nav-menus.php?action=edit&menu=26 then 26 is the menu ID.

And for submenu ID, according to the example, 127 is the submenu ID.