Your problem is not related to ports, or you wouldn't be getting the error messages back from Linkpoint.

Your problem is that your pem file is either:
- missing
- unreadable
- formatted incorrectly
- needing proper permissions
- blocked from external access
- double-spaced due to bad upload
- etc


What does your server's errorlog say? Every time you attempt a payment, Linkpoint comes back to your server and attempts to read your .pem file for security information. If it's blocked, your server's errorlog should say why.

Try accessing the file via your own URL, like this: https://your_site.com/blah/includes/modules/payment/linkpoint_api/123456.pem
- try with https and http
- what errors do you see?
- what's in the errorlog ?
- are you sure you don't have a .htaccess file blocking it ? ... in the / or /zencart or /includes folders?