This will guide you through the setup of the UniFi ac Lite AP.
Want to solve it permanently? Ensure your controller computer has a DNS resolvable hostname on your network(s) (you can ping it by name from your Unifi devices) then go into Settings on you controller, go to the controller section and ensure that your host name or IP address for your controller are entered intot he Controller Hostname/IP field, then check the Override inform host with Controller Hostname/IP like so:
If you don't also check the Override inform host with Controller Hostanme/IP box too, nothing changes. You have to do both - enter the hostname or IP addres and check the box!
Nine times out of ten when APs can't find the controller, your DNS is not pointing unifi to the IP address of the Unifi controller. By default the AP's fall back to trying to resolve the DNS name of unifi and if you can't issue a
ping unifi
from a command line on one of your APs you have ssh'd into and have it successfully resolve to your controller, that's your problem. Ensuring your IP address is in the field and checking the box should solve it if you can't control the DNS that is serving the AP's.
Also if your controller has more than one NIC configured, you will have a lot less issues adopting any Unifi kit if you ensure the override is enabled. I learned that one the hard way 😛