How to flash Official ROM on SAMSUNG GALAXY S6 EDGE
It looks like you Galaxy S6 Edge may have been rooted as I see current binary and system status are indicated as custom and that would explain not being able to OTA update.  I also know that some unlocked refurbished phone replace motherboards as long as the hardware fits.  Its my understanding that what is displayed in the download mode, i,n this case "G925L", will be the correct variant when choosing the proper firmware.
  I've rooted every Samsung I have ever owned and have never had the issues you described prior to factory reset.  It is my humble option that your phone may have been compromise  and suggest your change all your password accounts.  It would be easier is you did this after a successful boot rather than before as you'll face Google FRP lock.   Good luck