Quick update: being DrayTek support pretty useless, I was able to find the
solution, that is I accessed the Asus router via SSH and typed the following command:
ifconfig `nvram get wan0_ifname`:0 192.168.1.2 netmask 255.255.255.0
and now I can access the modem management GUI
No clue why I didn't need the
iptables command...
And finally I have to figure out how to create a script in order to load that command in case of a router reboot...