Setting up some new Ubiquiti Wifi Access points, on reboot the Wifi SSID would show for a bit then disappear. A look at the Radio’s on the devices came up as Auto (Disabled)

By Default, the Ubiqiti controller Site Setting has  “Enable Connectivity Monitor and Wireless Uplink” enabled by default. This means if the Access points can’t connect ( ping )  the gateways defined in Network they will no enable the SSID broadcast ( and disable Wireless )

You can disable this setting, but you probably want to update the Gateway in the Network details to the correct one , and make sure DHCP is running or set static and make sure it can communicate to the gateway