I cannot access my controller via the serial communication interface

The most common mistake is to use a USB-to-RS232 converter instead of a USB-to-UART converter. Logic levels must be 0 - 3.3V (5V tolerant). RS232 uses +/-12V logic levels and will not work. You can use a simple USB-to-UART converter such as TTL-232R-RPI. The best solution is to use an isolated converter such as the isolated USB-to-UART converter described by Waveshare.

You may also want to:

• Check that the GND pin is connected in addition to RX and TX pins
• Try swapping RX and TX pins.
• Install/reinstall the Virtual COM port driver: https://ftdichip.com/drivers/vcp-drivers/