Just got a reply from Zyxel support. Yes, it's not supported anymore. They could at least keep the latest firmware available in the download library...
They sent me this download link to get the firmware, though: https://drive.google.com/file/d/0B9Q3YT ... sp=sharing
I also found it here: https://www.zyxel.fr/support/download/200118_1
The French website file is 470AFK3C0.bin (MD5: 9717FB81D57E1C5406E29DE7D8A2478F), which seems to match what the NAS is trying to download. The google drive link they sent me is 470AFK3b1.bin (MD5: E9B7DC4EF37C6DA4E033079AC75096D7).
I believe the correct file is the *3C0.bin. Anyone who downloaded it before can confirm, please? Also, do you know if there's a difference in the firmware from country to country?