The MineOS plugin was changed to default to NAT for FreeNAS 11.3. Only the management port (8443) is forwarded though.
It also needs the ports of the actual Minecraft servers forwarded, otherwise one cannot connect a Minecraft client to the server.
The ports typically start at 25565. The official MineOS docker image forwards ports 25565-25570 to allow managing 6 servers. I suggest we do the same for the MineOS plugin.