The most recent firmware update disabled support for older receivers. The one you have bought might have been upgraded to that version.
As I have discovered this is my issue as well. Per Spektrum, the latest firmware for the DX4R PRO makes it so the radio cannot bind with original DSM receivers and can only bind at the 16.5ms frame rate with DSM2 receivers. Doesn't make sense to me why, but it is what it is. I now have to find a new home for my SR3520 and buy an SR2000 or its' new replacement if I want that ultra fast 5.5ms frame rate.
From Spektrum:
"This version will disable all RF protocols except DSMR and DSM2-16.5ms"