Ports can be a bit confusing with UDP because there's two ports for each device you use. One for "sending to" (Device Destination Port) and one for "listening" (Show Controller Source Port). These are often the same, but they can be different.
The V16Pro automatically "listens" to port 2639 (Fix number by Firmware) at all times and automatically sends back to the port it received a message on it will automatically send an "R" back to port it received it on.
Be aware adding a "0" into the "Show Controller" source port, it will automatically select a random port number.