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/support/download_landing.shtml
There you have access to downloads for most Zyxels devices, even old ones. Some downloads, e.g. recent firmware, require a MyZyxel login.
If you want to have quick access to files you should visit ftp://ftp.zyxel.eu
Remember, that for some devices, the firmware is only available for download, if you have an existing myZyxel account.