It is many many years since I had to play with real RS232 connections and one of the "challenges" was figuring out whether Tx should be connected to Tx or to Rx - because some connecting cables swapped the wires internally and some did not. And some devices expected the wires to be swapped and some did not.
Can you confirm that the USB-RS232 dongle works when connected to the radio with just Tx Rx and GND and NO OTHER connection. If it does it will greatly simplify further troubleshooting.
That will almost certainly require you to connect between the dongle and the radio with some temporary wires.
Can you also confirm (posting a photo of a simple diagram would be best) how the Tx Rx connections from the dongle connect to the Tx and Rx connections of the radio. And if other connections are essential to get it to work then please give those details as well.
...R