It appears that there are two ports used by the FLEXlm server. They can be determined from the log file of the license server and set by editing the license file.
One of the ports will by default be in the 27000-27009 range by default. This can be manually set by the end user by adding a port number to the end of the SERVER line of the license file.
The other port appears to default to 4508 (for me at least) and can be manually set by adding the key/value pair ‘port=#####’ (where ##### is some TCP port) to the end of the VENDOR line of the license file.
Ex.
SERVER Leviathan 00144f23a434 2878
VENDOR frantic port=2879
Which sets up the server to use TCP ports 2878 and 2879.
Remember to restart the server after making these changes, and check the log file to confirm them. I hope this answers your question.