The plugin hasn't been updated for a while, so not sure if it is still under development?

I've tried it, but can't get it to work. October is installed on a server using DirectAdmin. I've put the servers IP in the second domains A-record and created a domain pointer inside DirectAdmin to connect the domain with the hosting account. In the plugin settings, I created the second domain and assigned a theme to it. However, when entering the second domain in my browsers address bar, I only end up being forwarded to the main domain.

Any help would be great.

I have the same problem. The registered domain is routed to the primary domain.