add_filter( 'wp_get_nav_menu_items', 'wpcb_add_product_subcats_to_menu' );
if ( ! function_exists( 'wpcb_add_product_subcats_to_menu' ) ) {
	/**
	 * wpcb_add_product_subcats_to_menu.
	 *
	 * @see https://wpcodebook.com/snippets/automatically-add-woocommerce-product-subcategories-to-categories-in-menu/
	 */
	function wpcb_add_product_subcats_to_menu( $items ) {
		if ( is_admin() ) {
			return $items;
		}
		$children_order = count( $items ) + 1;
		foreach ( $items as $item ) {
			if ( 'product_cat' == $item->object ) {
				$children = get_terms( array(
					'taxonomy'     => 'product_cat',
					'hide_empty'   => false,
					'parent'       => $item->object_id,
					'limit'        => -1,
					'orderby'      => 'title',
					'order'        => 'ASC',
				) );
				if ( ! empty( $children ) && ! is_wp_error( $children ) ) {
					$children_submenu = array();
					foreach ( $children as $child ) {
						$tmp_item = new stdClass();

						$tmp_item->ID                      = $child->ID * 10000;
						$tmp_item->post_author             = $item->post_author;
						$tmp_item->post_date               = $child->post_date;
						$tmp_item->post_date_gmt           = $child->post_date_gmt;
						$tmp_item->post_content            = '';
						$tmp_item->post_title              = '';
						$tmp_item->post_excerpt            = '';
						$tmp_item->post_status             = 'publish';
						$tmp_item->comment_status          = $child->comment_status;
						$tmp_item->ping_status             = $child->ping_status;
						$tmp_item->post_password           = '';
						$tmp_item->post_name               = $child->post_name;
						$tmp_item->to_ping                 = '';
						$tmp_item->pinged                  = '';
						$tmp_item->post_modified           = $child->post_modified;
						$tmp_item->post_modified_gmt       = $child->post_modified_gmt;
						$tmp_item->post_content_filtered   = '';
						$tmp_item->post_parent             = 0;
						$tmp_item->guid                    = $child->guid;
						$tmp_item->menu_order              = $children_order;
						$tmp_item->post_type               = 'nav_menu_item';
						$tmp_item->post_mime_type          = '';
						$tmp_item->comment_count           = $child->comment_count;
						$tmp_item->filter                  = 'raw';
						$tmp_item->db_id                   = $tmp_item->ID;
						$tmp_item->menu_item_parent        = $item->ID;
						$tmp_item->object_id               = $child->ID;
						$tmp_item->object                  = 'product_cat';
						$tmp_item->type                    = 'taxonomy';
						$tmp_item->type_label              = '';
						$tmp_item->url                     = get_term_link( $child->term_id );
						$tmp_item->title                   = $child->name;
						$tmp_item->target                  = '';
						$tmp_item->attr_title              = '';
						$tmp_item->description             = '';
						$tmp_item->classes                 = array ( 0 => '' );
						$tmp_item->xfn                     = '';

						$children_submenu[] = $tmp_item;
						$children_order++;
					}
					$items = array_merge( $items, $children_submenu );
				}
			}
		}
		return $items;
	}
}

You can add this snippet to your (child) theme's functions.php file.