In highly contested environments, wireless controllers can have difficulty setting access point channels to avoid interference. When this happens, it may be necessary to disable automated optimization and statically assign the channels.
DFS channels are a set of 5GHz channels that are commonly disabled in a default configuration. It is recommended in most circumstances to enable these channels, as the majority of the 5GHz bandwidth is contained within these bands. For more reading, see our article Understanding DFS Channels.
Instructions from UniFi Controller running firmware version 7.1.67.