UniFi devices retain their running configuration post-adoption. The controller is not required to be running for the devices to forward traffic. If the controller is restarted it will not restart your access points or other devices. All configurations as well as your internet connectivity will continue working as usual, but you will lose anything running on the Controller directly instead of a device, for example, statistics, reporting, manually applied user groups, and most importantly, the Guest Portal.
When the Guest Portal feature is enabled, the controller is functioning as a captive portal. When a UniFi access point (UAP) cannot reach the controller, it goes into "SELFRUN" state. In this state, it will not redirect guests to the portal since it is not reachable, so the UAP will automatically allow guests to use the network without redirecting. Moreover:
- The guest access policies will still be effective, along with the restricted subnets feature.
- The user group configuration associated with this WLAN will still be effective.
- When the Controller comes back online (and the UAP goes into MANAGED state), the Guest Portal redirection feature will restore automatically.
If you wish to configure the "SELFRUN" state, please see this article. If you would like to know more about setting up Guest Networks (including the Portal), please see: UniFi - Guest Network, Guest Portal and Hotspot System.