To enable WAN ping on the ZyXEL VMG1312 you will need to access the router interface through a web browser. To do this make sure you're on a device that is connected to the router, open a web browser, and then type the IP address of the router (192.168.1.1 by default) into the address bar then press enter. You will then be asked to log into the router.
By default the username is admin and the password is 1234, but the password should have been changed during the setup of the router.
Once logged into the router hover over the "Maintenance" option on the bottom menu and the select the "Remote MGMT" option from the popup menu.
Once you are on the "Remote MGMT" screen tick the box to the right of "ICMP" and under "WAN", then click apply.
WAN ping should now be enabled. If you wish to disable WAN ping you simply follow the instructions once more but make sure the box is unticked rather than ticked.