Wordpress error 504 - hot to troubleshoot and fix

1. In the wordpress root directory, greate file info.php with following php code
<?php phpinfo(); >
2.Access this file
http://[web site]/info.php
if the page loaded but wordpress still response with 504 move to next step. if not - try to restart php service and apace or Nginx
3. Enable debug mode in wp-config.php
define('WP_DEBUG', true);
try to access website and follow the errors to try to understand what caused it. if still 504 error without explanation, move to next step
4. Start to disable instaled plugins one by one. First, set the order of installed plugins by date. 
Very likely one of last installed plugins cause that error

in my case, paid-memberships-pro cause the error.