To enable remote connections 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.
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 on the Remote MGMT page tick the WAN option on the HTTPS row, then save the changes at the bottom of the page.