When you set the WLC to L3, that means the ap needs to have an ip address. Management and ap-manager needs to be on the same subnet and the time has to be set on wlc. Place the ap on the same subnet as the wlc management and ap-manager's interface. Make sure there is a dhcp scope on that subnet... no need for the options on the scope. The ap will get an ip address and find the wlc. Then you can move the ap to the correct vlan and see if the ap finds the controller. Doing it this way, tha ap will know of the wlc and should have no issues finding the controller again.