I recently navigated the process of finding and installing the latest firmware for the D-Link DIR-853 A3, specifically looking to resolve some intermittent connectivity drops on the 5GHz band. Here is my experience regarding the file integrity and the update itself.
D-Link DIR-853 revision A3 , firmware updates are primarily available through official D-Link FTP repositories or alternative community firmware sites if official support for a specific regional "ET" variant is limited. Official Firmware Download Links dir853 et a3 firmware update link