There are multiple ways to get the latest firmware or documentation for your Zyxel devices.
The easiest way is our Zyxel Download Library.
Some products from the SP branch can be found here:
There you have access to downloads for most Zyxels devices, even old ones. Some downloads, e.g. recent firmware, require a MyZyxel login.
Remember, that for some devices, the firmware is only available for download, if you have an existing myZyxel account.