I don’t know if I have understood your problem correctly but to flash the firmware on some Matek MR24-30 for SIK telemetry the following procedure was successful for me.
You must first set the device to DFU mode. When the message ‘DFU in FS Mode’ appears in the device manager under USB Devices, call up the mLRS Web Flasher and click on the desired firmware version and the firmware in the corresponding selection list. The Web Flasher automatically retrieves the firmware file. You do not need to download anything beforehand.
Then click on ‘Connect’ and the selected firmware will be loaded onto the device. If the flashing was successful, the Web Flasher displays a message like: ‘Flash successful. Go flying.’
If you did not see this message, I am not sure if the process was successful. It also seems to be important that TX and RX have the same firmware version.