This device can be flashed with tasmota. Ensure that GPI00 is set to ground while powering up the unit (then GPIO0 can be unplugged). VCC (3,3V) and ground can be plugged either on the board or on the controller. Connect TX/RX pins and set the serial speed to 115200 bauds then use the serial flash procedure. If you hold the wires with the hand (just as i have done), the GPIO0 pin must be continuously set to the ground, since you are likely to restart the unit (GND and VCC pins are right next door).