ATmega16u2 DFU does not work

Hello guys,

I´ve tried to flash my ATmega16u2 of my Arduino (Elegoo) Uno Rev.3 with an Firmware which is called UnoJoy (UnoJoy) to turn a Arduino into a Joystick, but i can´t get my ATmega16u2 into DFU Mode (Device-Firmware-Update Mode).

When I briefly connect the reset-pin and the gnd-pin my computer shortly displays that the arduino is plugged off which is correct but after a few seconds it recognizes it as a normal Arduino Uno but it should be shown as Arduino DFU.

While the two pin are connected the arduino doesn´t show up.

Can anyone help me, why the Arduino doesn´t get into the DFU Mode?
I would appreciate about any help.