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.