You found menu 'enter in DFU mode' on V2Plus4 device? Or no?
For enter in Bootload mode (used in device CPU not have DFU mode, it use bootloader) need hold right button (if connectors up) and power on device
PS be careful on use ST-Link programmer for write firmware (use only QT or NanoVNA-App), at start adress written bootloader, this boodloader use CPU unique ID for run, and if damage it you brick device.
