UAP AP Port Conf

General Conf: (you must disable this)

no vlan dot1q tag native

The management interface or SSID must not be tagged but all others must be for this to work. 

Port Conf:

interface GigabitEthernet1/3
description WAP0
switchport trunk encapsulation dot1q
switchport trunk native vlan <LAN Vlan>
switchport trunk allowed vlan <LAN Vlan>,<GUEST Vlan>
switchport mode trunk
spanning-tree portfast trunk
end

In the conf above the native vlan must also be listed in the "allowed vlan" conf line. 

UBNT Controller: More info here https://help.ubnt.com/hc/en-us/articles/219654087-UniFi-Using-VLANs-with-UniFi-Wireless-Routing-Switching-Hardware#UAP