@Arva
I can only offer the same advice given to Taikeido in Post #308 and it seems you've already attempted to flash the underlying RGN, resulting in that error message, but using a cure fw.

About preboot mode: Your device has defaulted to preboot/boot block/garmin mode or it would not be visible in Device Manager. Most devices which have a faulty 'unit software' (firmware) will indeed default to that mode as your fenix has done (sometimes "System Software Missing" or similar message is displayed on the device screen).

Usually a 'soft' bricking occurs immediately after a firmware or files update, meaning it happens at the very next boot attempt after the update. If such a bricking is caused solely by firmware, it can usually be fixed by simply re-flashing an original (NOT cured) fw to overwrite the corrupt data and without actually having to follow a process to access preboot because it's already in that state.

Unfortunately, suddenly failing like that in normal operation can be strongly indicative of a hardware failure rather than a firmware problem (i.e., a 'hard' bricking, not repairable by software methods). Usually that's due to flash memory damage, not effectively repairable by users.

The error message you received can also be because the device/pc connection was lost, even momentarily:
* Try another USB cable, more than one if several are available;
* Re-read the suggestions in Post #308 to be sure to make the RGN fw file correctly, and,
* Be persistent in trying to get it to flash, it may take numerous attempts to flash a flaky device which moves in an out of preboot.