It works!
If someone needs help to restore stock firmware, this is the procedure that I did.
cd /tmp
wget "https://d3jal3boi407dg.cloudfront.net/mycloud/wsm20/latest_firmware_info/s3_file/1652350021659/V1.00(ABZF.4)C0.bin"
mv "V1.00(ABZF.4)C0.bin" "zyxel.bin"
[ "$(md5sum zyxel.bin | awk '{print $1}')" == "b601f1ee260460e4107345003f82b7f4" ] && echo "Firmware check passed!" || echo "Firmware is corrupted, download it again!"
mtd write zyxel.bin Kernel2
echo -ne "\x02" | dd of=/dev/mtdblock7 count=1 bs=1 seek=4 conv=notrunc
reboot
I recommend running one command at a time, to check if it was successful.
If it says Firmware is corrupted, download it again! do again the wget.