There are multiple ways to get the latest firmware or documentation for your Zyxel devices.
The easiest way is our download library at https://www.zyxel.com/de/de/support/download_library.shtml
There you have access to downloads for most of Zyxels devices, even old ones. Some downloads, e.g. brand-new firmware, require a MyZyxel login.
Remember, that for some devices, the firmware is only available for download, if you have an existing myZyxel account.