While you are connected to the Zyxel router's WiFi, open any browser and navigate to 192.168.1.1. The default username admin and the password 1234.
1. Select the Settings icon (two gears) on the left side of the screen.
2. Select Wireless Settings LAN 2.4G.
3. Select the More AP tab.
4. Select Edit on the right-hand side of Row 3.
5. Check the box at the top to Activate.
6. Enter a name under Name (SSID).
7. Set password under Pre-shared Key and then select Apply.
It will take approximately one minute to apply at which point the page will refresh. You may have to reboot your router for the changes to take effect.
If you do not want devices to be able to communicate with other devices on your network, enable the Intra-BSS blocking option in the settings.