If you want to know why you can´t upgrade the latest firmware, this article gives you the clue.
There are two ways to upgrade a firmware on a NAS: Upgrade via internet or through a locally saved 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 FTP server: ftp://ftp.zyxel.com. Afterwards, you only need to click on "Upgrade".