The ZyXEL VMG1312 has a web interface and a Command Line Interface (CLI). The CLI can be reached by telnet or ssh. Usually the CLI is not needed to be used as all the config changes can be done from the Web interface. However, here are some useful commands for the CLI.
Show running processes and CPU/memory information:
Change LAN address to be 192.168.100.1:
lan config --ipaddr primary 192.168.100.1 255.255.255.0
Note: Check your DHCP settings when changing the LAN address, the DHCP range may well change itself when you change the LAN address this way anyway.