I am working in a new project were the vendor installed over 10 Access point model AIR-AP1852E on a WLC5508 with Software Version 8.2.100.0
but i can’t configure those AP in Flexconnect Mode.
Cisco documentation Confirm that the available modes are “Centralized local”, “Standalone”, “Sniffer”, “Cisco FlexConnect”, “Monitor”, “OfficeExtent” and “Mesh”.

but it only show you the “Local” & “Sniffer” so i will guide you how to fix it

so i login to Cisco web site and downloaded the Newer image 8.2.130.0 and Save it to file

unless you have a service contract you will not be able to download it
then Login to WLC

now to upload to the WLC
go to Commands – Download File and Fill the Detail of your TFTP and WLC File Name and then press the Download Button
you can see in your TFTP it Start to upload

After it finish the Upload it will ask you to Reboot
I Choose Save and Reboot
Now you see it change and show all Mode

Now go to Flex Connect and Check the Box of VLAN Support
and put the Native Vlan ID in your Branch – Apply
Then Press Button Vlan Mappings
Then Configure your SSID with the Proper VLAN and
Note. even if you are Having this VLAN in another Site it doesn’t matter as long your Access Point is Flexconnect then it will take the branch IP Addressing 

and Finally as you see i took my branch IP Address

;D