Nas542 updated for version V5.21(ABAG.9).

Kissi

 A nyomtatószerver azóta eltűnt.

ikubuf

The CUPS has been removed from the NAS package. 
You can check the release note.

BTW, you can refer to this article to downgrade the NAS firmware.
https://community.zyxel.com/en/discussion/5948/nas326-firmware-upgrade/p2
But be aware of the vulnerability of CUPS.
There is some issue with php package, so if you would like to do downgrade, downgrade to V5.21(ABAG.7)C0

Kissi

Instead of ftp://ftp.zyxel.com/NAS326/firmware/, https://wwwzyxel.com/global/en/support/download is displayed, from which the previous firmware cannot be downloaded.

ikubuf

You can get the firmware on FTP site by using filezilla or other FTP client to access the FTP site.
Try with the URL: ftp.zyxel.com

Kissi

NAS message
The NAS firmware cannot be downgraded, please select a newer version of binary (BIN) upgrade file from your computer.

ikubuf

You can refer to this article to downgrade the NAS firmware.
https://community.zyxel.com/en/discussion/5948/nas326-firmware-upgrade/p2