Reviving a bricked AT&T branded nighthawk mobile router

Has anyone experience in flashing a MR1100 back to life? All the device does is showing the NIGHTHAWK logo, then after some seconds the AT&T logo appears... and then ther loop starts again. Finally after about 5 of such loops the screen turns off (I noticed, it shows a blue/orange flashing LED that turns into steady orange after some minutes - the main display remains black).

Can anybody help me to unbrick this router?
Thanks in advance :)