If you want to know why you can´t upgrade to the latest firmware, then this article will give you a clue.
There are two ways to upgrade the firmware on a NAS:
Upgrade via the internet or locally with the firmware file.
To check for the latest firmware, you can navigate to "Control Panel > System > FW Upgrade > Latest Firmware Check" and click on "Check Now". The Firmware can be applied with the "Upgrade Now" Button. If the check fails, please make sure that the NAS has the correct gateway and DNS set up. These can be checked on "Control Panel > Network > TCP/IP"
If you want to upgrade the firmware locally, you can navigate to "Control Panel > System > FW Upgrade > Manual Firmware Upgrade" and select the .bin file, which can be found in the .zip file on our Zyxel download library.
Afterwards, you only need to click on "Upgrade".