re: Bought "demonstrational" Nokia Lumia 1520
Had this to me back when the Samsung Focus was relevant. It had the AT&T demo software running, and after many hours of researching, turns out that AT&T demo software cannot be flashed over by anyone other than AT&T due to some special decryption software they use.
@OP, I know this is regards to a Lumia 1520, so you might get luck and be able to flash over non-demo firmware, but before you attempt to do that, which could cause additional issues including bricking the phone, I would definitely contact Nokia and see if they will work with you to get the demo device back and send you a Lumia 1520 that has the correct firmware. If for any reason Nokia does not help you, then I would try flashing it as a last resort only.
Good luck, and be sure to let us know how it turns out.