functions.php

<?php

function my_custom_submenu() {
  global $post;
  
  $menu_items = wp_get_nav_menu_items('Menu');
  $current_menu_id = 0;
  
  // get current top level menu item id
  foreach ( $menu_items as $item ) {
    if ( $item->object_id == $post->ID ) {
      // if it's a top level page, set the current id as this page. if it's a subpage, set the current id as the parent
      $current_menu_id = ( $item->menu_item_parent ) ? $item->menu_item_parent : $item->ID;
      break;
    }
  }
  
  // display the submenu
  echo "<ul id='supplementary_menu'>";
  
  foreach ( $menu_items as $item ) {
    if ( $item->menu_item_parent == $current_menu_id ) {
      $class = ( $item->object_id == $post->ID ) ? "class='current_page_item'" : "";
      echo "<li {$class}><a href='{$item->url}'>{$item->title}</a>";

      $sub_menu_items = [];

      foreach ( $menu_items as $sub_item ) {
        if ( $sub_item->menu_item_parent == $item->ID )
          $sub_menu_items[] = $sub_item;
      }

      if ( $sub_menu_items ) {
        echo "<ul>";

        foreach ( $sub_menu_items as $sub_item ) {
          $class = ( $sub_item->object_id == $post->ID ) ? "class='current_page_item'" : "";
          echo "<li {$class}><a href='{$sub_item->url}'>{$sub_item->title}</a>";
        }

        echo "</ul>";
      }

      echo "</li>";
    }
  }
  
  echo "</ul>";
}