How to add a custom menu to a specific location in Wordpress

You should instead use the wp_nav_menu_objects filter, which allows you to modify an array of items instead of strings.

example:

add_filter( 'wp_nav_menu_objects', 'restructure_menu_links', 10, 2 );

function restructure_menu_links( $items, $args ) {

    $new_links = array();

    $label = 'Lorem Ipsum';    // add your custom menu item content here

    // Create a nav_menu_item object
    $item = array(
        'title'            => $label,
        'menu_item_parent' => 0,
        'ID'               => 'yourItemID',
        'db_id'            => '',
        'url'              => $link,
        'classes'          => array( 'menu-item' )
    );

    $new_links[] = (object) $item; // Add the new menu item to our array

    // insert item
    $location = 3;   // insert at 3rd place
    array_splice( $items, $location, 0, $new_links );

    return $items;
}