This page explains the necessary configuration for Mist Systems Access Points and external Captive Portal with RADIUS authentication.
In the Mist Web GUI, create a Guest WLAN to use your captive portal pages. This action requires an administrator account Network Admin privilege or higher.
To start, create a new WLAN by navigating to Network -> WLANs and selecting the Add WLAN button on the upper right.
Under the Security section, select the desired WLAN security mode
Click Create to create the new WLAN on your site. The API Secret will automatically be created for you.
Go back to the IronWiFi console and paste the API Secret to Controller Password on the Captive Portal settings page.