https://jrtashjian.com/2017/05/highlighting-parent-page-in-wordpress-menus/

gistfile1.php

<?php

/**
 * Add the 'current-menu-ancestor' class to a parent page menu item where the child page is not part of the menu.
 *
 * @param array   $classes The CSS classes that are applied to the menu item's <li> element
 * @param WP_Post $item    The current menu item
 *
 * @return array The filtered CSS classes.
 */
function active_parent_in_menu_for_page( $classes, $item )
{
    global $post;

    if ( !is_page() ) {
        return $classes;
    }

    $page_parents   = get_post_ancestors( $post );
    $topmost_parent = array_pop( $page_parents );

    if ( empty( $topmost_parent ) ) {
        return $classes;
    }

    if ( $topmost_parent == $item->object_id ) {
        array_push( $classes, 'current-menu-ancestor' );
    }

    return $classes;
}

add_filter( 'nav_menu_css_class', 'active_parent_in_menu_for_page', 10, 2 );