It's a bit odd that they have removed a large amount of devices from their download library links.
The 8924 and 1312 specs/support pages have been gone a while but you could still search for the device and find firmware links.
Still about a dozen firmware versions for the 8924 on their ftp site.
You need to know the link as there's no way to find it through Zyxel's website.
ftp://ftp.zyxel.co.uk/They even have a non ISP firmware for the apparently ISP only VMG8825-T50K discussed above.
ftp://ftp.zyxel.co.uk/VMG8825-T50K/firmware/Which is more than they provide for some other models issued to ISP's in parts of South America and Egypt.
With a few of the VMG series models used by some members here on kitz, Zyxel would often help providing the Supervisor password without having to decrypt it yourself.
However they would ask for the router serial number and if you provided the serial number of an ISP supplied device support would end at that point, referring you back to your ISP.
If you provided a retail model serial number they would provide the Supervisor password and even offered to register the devices warranty for you.