try the following:

set the stick to power OFF plug in USB (it will automatically power ON) look in M5Burner (or in your computers device manager) if the serial port appears. If not there is a driver issue on your computer or your USB cable might have broken wires (once happened to me) if the port appears burn the factory test or UiFlow

I suppose there no need to put the stick in any special mode to burn the firmware. the internal USB chip will do that for you while the programm downloads. Once the UiFlow Firmware is running you have to set the mode for wifi or usb if not already done while burning firmware.