I had proper NAT rules in place and proper firewall forward rules in place, and the solution for me was this: https://forums.plex.tv/discussion/comment/1450524#Comment_1450524
In summary, I had to connect to the box ‘locally’ (in my case using an ssh tunnel connection to the box and a SOCKS5 proxy), after which it showed I was connected just fine.
I then checked the server on another local machine without the proxy running and connecting directly at ip_address:32400, and then it showed I was connectable from the outside.
My rules firewall rules are basically what’s outlined here: http://forum.mikrotik.com/t/firewall-rules/69251/1 with an added top level forward chain rule to allow tcp port 32400.
Use something like http://www.canyouseeme.org/ to see if your port 32400 is accessible.