As you are facing the radio, which side of the radio are you plugging the RJ-45 jack? if you are plugging it into the left side securely (the side closest to the power button), then the only thing I can think of is,,, the RJ-45 jack termination is defective.
Keep in mind, with or without the RJ-45 connection being made to the radio, Windows Device Manager will still recognize the Silicon Labs CP210x USB to UART Bridge while it is connected to the PC's USB port. I just did that on my radio. read the radio, and then got the error message "Failed connection ! Please check if powered on or well connected to the com port!"