Custom order by

I am looking for a way to order by attributes from low to high and high to low. I tried adding this to my functions.php but it will order by something (always the same order) but no idea what it orders by.

So this is what i am working with at the moment, so far it comes up in the orderby function from woocommerce. And it tries to sort something but not the attribute i am trying to sort.

‘nominaal-koppel-nm’ is the attribute that in this case i am trying to sort by number. A value of an attribute like this would look like 1.5 [Nm] so not sure if it actually counts as a number since there are letters behind it.

//add new options to $sortby var passed into filter
function custom_woocommerce_catalog_orderby($sortby) {
    $sortby['koppel_asc'] = "Sorteer op Koppel H-L";
    $sortby['koppel_desc'] = "Sorteer op Koppel L-H";
    return $sortby;
}

//Attach our function to the filter hook:
add_filter('woocommerce_get_catalog_ordering_args', 'custom_catalog_ordering_args');

//Function to handle choices
function custom_catalog_ordering_args($args) {
    global $wp_query;
    // Changed the $_SESSION to $_GET
    if ($_GET['orderby'] == "koppel_asc") {
        $args['meta_key'] = 'nominaal-koppel-nm';
        $args['orderby'] = 'meta_value_num';
        $args['order'] = "ASC";
    } else if ($_GET['orderby'] == "koppel_desc") {
        $args['meta_key'] = 'nominaal-koppel-nm';
        $args['orderby'] = 'meta_value_num';
        $args['order'] = "DESC";
    }
    return $args;
}

I have seen so many questions really similar like this on the internet but no one can really explain how to get this done.

Thanks for your time ,

Sjoerd