There are two ways to get the file:
The DWR-M920 may have different hardware revisions (e.g., A1, B1, C1). Installing firmware for the wrong revision will brick the router.
Updating the firmware on your D-Link DWR-M920 ensures you have the latest security patches, bug fixes, and performance improvements.
This is the most common point of failure. D-Link devices are manufactured in hardware "Revisions" (e.g., Rev A, Rev B).