I had the exact same issue as you with this device. It is not obvious what to do and I ended up opening a TAC case to have it resolved. The big thing to remember is that the WLC cannot hand out the DHCP address to the AP, and the DHCP request is going to pass out the ap-manager port and vlan if it is configured.
Below is the answer of the TAC engineer and it works:
To configure this please open the GUI/ go to Controller/ Interfaces/ ap-manager/ under DHCP information configure the IP address of the primary DHCP server, you should also be aware that the APs plugged into port 7 and 8 are going to get an IP address from the same range that the ap-manager, so in order to this to work both the ap-manager and the DHCP should be on the same subnet.
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
This is the start of a display filter cross reference between Wireshark
and OmniPeek. The 1st installment is a table of advanced filters. More
filters will be added as time allows. It is a living doc, so check back
for changes every so often Please feel f...
I have created a Powershell script to automatically add a Wireless Guest
User on Cisco WLCs. (tested on 2500 Series) The script should be
completely self explanatory. Prerequisites: Powershell SNMP Module
(Install-Module -Name SNMP) SNMP Write Access to y...