$args = array(
    'post_type'  => 'product',
    'meta_query' => array(
        'relation' => 'OR',
        array(
            'key'     => 'nazwa_pola_acf',
            'value'   => $_POST['search'],
            'compare' => 'LIKE',
        ),
        array(
            'key'     => 'inne_pole_acf',
            'value'   => $_POST['search']
            'compare' => 'LIKE',
        ),
    ),
);
$query = new WP_Query( $args );