Third party plugins and mods sometimes require extra ports in order to function. However in order to use extra ports you must allocate them which can be done in the nitro panel.
Click the Network menu and then select Create Allocation. This will randomly create a new port for you to use.
The port that isn't primary is now your new allocation. Plugins like dynmap will have a configuration file that lets you enter in the correct port.
Want to use a specific port instead of random one? Contact support and request a port modification free of charge.
Customers who don't have dedicated IP's may be limited to the ports they can use.