I'm using a 6.24 Mikrotik router. I just successfully got Plex working, using a simple DST-NAT as you did.
1) Under firewall > nat, add a new rule. Mine looks like this:
(Under the assumption that my Plex PC inside my LAN is on 192.168.1.50 and my public WAN address is 18.104.22.168)
chain=dstnat action=dst-nat to-addresses=192.168.1.50 to-ports=32400 protocol=tcp dst-address=22.214.171.124 dst-port=32400 log=no log-prefix=""
2) Go into Plex Media Server
Then click the 'Connect' button. When you tell the server to connect, it will "phone home", allowing access from outside. Now, anyone logged in with your Plex account can access this server, while your public IP stays the same. You can troubleshoot by watching the counter on the DSTNAT rule.