Unit not visible for a long time in updater and not possible to be flashed with .rgn firmware scares me. This could be due to problem in the memory, boot or lets hope you are not doing something in the right way...Hope it is the second one. Try different ways and see if you can try to flash the .rgn firmware. Unfortunately, I am not very fammiliar with 2xxx series. As I wrote the best way is through .rgn as .gcd needs more time to be read from the boot, checked and flash the correct regions.