Hummm…
So first things first. when you connect the USB cable to your PC adds another COM port right? You should look on your PC Device Manager. Assuming that it does…are you connected to that port right?
On OpenCNCPilot turn on debug messages to see the message exchange responses:
If nothing is happening try to reinstall Marlin like you did once. Or in alternative reinstall the original Robin Nano firmware.
If the bootloader somehow got corrupted you will need to reaploaded with an ST-Link although that should not be the case. The boot loader wright the firmware to a safe position and the Flash eeprom is far way from all that.
Let me know how it went.