NW Doc

You cannot add a new listening port to your Terminal Server via the GUI (Terminal Server Configuration in Administrative Tools) because there is no option for changing the listening port via the GUI.

Create the new RDP listening port via the registry.

Run Regedit.exe on your Terminal Server.
Navigate to the following key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp

Export the entire key to a .REG file by selecting the key (click on it...) and going to File > Export.

Give it any name you want.
Edit the .REG file you've just created and change the name of the key at the 3rd line of the file to something like:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp-New

Do not double-click on the .REG file, instead right-click it and choose Edit.
In the .REG file find PortNumber and change it to whatever port you want. Make sure you write it in hexadecimal format. For example, for 3390 you must enter D3E (use calculator in scientific mode to easily convert decimal to hexadecimal format).
Import the .REG file back to the registry by double-clicking on it.