We've been having a hell of a time figuring that out. My gut feeling is that it's stymying Uniden as well (making it work outside a firewall).
Port 554 if I recall correctly is the "control" port for, shall we say, data functions like pause, play, etc.
Everything that I've seen discussed on the forum to date says that the audio port(s - two of them) changes every time the connection is made. The RTSP client/server negotiates the pair of ports upon connection, and the standard allows for the ports to be drawn from a very large pool (something like 1024 to 65520).
There are some folks who, I think, made it work after a fashion by streaming from the scanner to VLC on a home computer, and then streaming VLC out to the world on a static, known port. IIRC you might end up losing the ability to control the scanner using this method, but you can get audio outside of your firewall into the Internet proper.
All that said - the current version of the 536 and Siren app does not support control/audio outside your home network natively.