How to restrict search on a certain page to only return results against custom taxonomies?

You did not return the new $query; that’s why it’s not working for you.

$query = new WP_Query( $taxquery );    
return $query;

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)