Latest firmware for NAS542

I purchased a secondhand NAS542, which is running V5.21(ABAG.13). I discovered there is a ABAG.14 update that fixes several vulnerabilities: https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-multiple-vulnerabilities-in-nas-products-06-04-2024%2C however the link to download the firmware on this page seems to be dead. I also found references to an FTP share with the firmware (ftp://ftp2.zyxel.com/NAS542/firmware/), but this FTP seems to be empty.

Is there any way to obtain the V5.21(ABAG.14)C0 firmware update file?

Thanks in advance.