have you tried the "reboot" command when logged in as Supervisor via telnet/ssh?
Zyxel doesn't send any supervisor credentials to end/home users like me. Anyway, to close this thread, I finaly found a workaround called ssh tunneling (this can be used even if you don't have https access from WAN due to TLS error) and with this I mirrored the http localhost site of the gateway to the localhost of my mobile. Then, I just logged in and issued a reboot from web interface as normal.
If it was not for ssh tunnelling, I would never have done it but I would prefer to have elevated rights to my product anytime than having to rely to workarouds like this.