Yes the front page is set as static.
Also I have tested below code with (EP_ROOT, EP_PERMALINK, EP_PAGES, EP_ALL) but it doesn’t work 🙁
function productpage_rewrite_endpoint(){
global $wp_rewrite;
add_rewrite_endpoint(‘productpage’, EP_ALL);
$wp_rewrite->flush_rules();
}
add_filter(‘init’, ‘productpage_rewrite_endpoint’);