Add a Location
A new site can be created by clicking on the switch sites button and adding a new location.
Select Zyxel Unleashed, provide a name for the location and click on Save.

Get AP MAC addresses from Nebula
Login to Nebula cloud and navigate to Access points tab. Note down the MAC address of all your APs. You can also export the APs as a CSV file and export to Spotipo.

Add Zyxel APs to Spotipo
Navigate to Zyxel Aps tab on the left and Click on New.

Enter details of the AP and click on Save.

After adding, spotipo will generate NAS identity and Radius Secret. Please note it down.

Configure SSID Authentication in Nebula Cloud
Navigate to SSID overview in Nebula Cloud. And press the Edit button in authentication section

Select Sign in method as Sign On With My Radius Server
Use 34.77.150.10 and 35.205.248.64 as IP of first and second radius servers respectively
Use 1812 as the port number and use the secret generated in the previous step
Configure NAS identifier with value created in previous step.
Enable account and fill the radius server values as 34.77.150.10 and 35.205.248.64
Use 1813 as port number
Configure Walled Garden
Enable Walled garden range and enter below values
logme2wifi.com
*.facebook.com
*.fbcdn.com
www.facebook.com

Enable Guest Network
In the SSID overview page, enable Guest Network Setting. Remember to press Save.

Configure Captive Portal Settings
Navigate to Captive Portal Settings page in Nebula cloud.

Enable use URL in External Portal Setting
Use https://logme2wifi.com/zyxelnebula/guest/ as the URL
Change Captive portal Behaviour as "To promotion URL" and use https://logme2wifi.com/zyxelnebula/postauth/ as the URL

Save and test
Save the settings and wait few minutes for it to be active. After that try to connect a device to Guest SSID and you should be seeing the splash page.