My F3 (16GB) stopped working - only the red LED is lit, it is not recognized by my computer over USB. However, when I connected a cable to the ETH ports, the activity LEDs came up.
I tried:
Fresh SD card, power over DC and USB, pressing the buttons to enter download mode, and using the DIP switch to enter download mode.
You can try using the network port-based SSH to control the board instead. The IP address acquisition can be done on the upper-level router.
However, I’m not sure if the USB interface of the board or the serial port cable is broken. You can try replacing the cable and the PC connection and see if it works.