My first post as I’ve been desperate for help but found solution thanks to this post:
https://forum.ghost.org/t/digital-ocean-mailgun-setup-issue/28091/3
Besides that make sure that you don’t copy host details from Ghost documentation. Before, check that correctly with your SMTP setup page on Mailgun website. Mine was smtp.eu.mailgun.org instead of smtp.mailgun.org
So I come across two issues but definitely port 587 works with TLS.
Now can focus on a theme 
Hope this will help others too.