Please connect the USB-UART bridge with Duo as below:
Duo --- USB-UART bridge
TX --- RX
RX --- TX
GND --- GND
Open a serial terminal (e.g. PuTTY or Tera Term) and open the comport for your USB-UART bridge. Set the baudrate to 115200.
When your Duo run up, you can see the debug message printed on serial terminal.