We have a FlexConnect AP in a remote site. The AP has successully joigned the WLC 5508 (runnig 188.8.131.52) through WAN connection, and broadcats an enabed WLAN. However client do not receive DHCP leases and then can not connect properly to the related WLAN. The AP is located behind a router witch NATs local IP to a public IP through its WAN interface.
Wireshark traces on the wireless interface of a laptop, show a lot of DHCP DISCOVER.
If you are doing FlexConnect Central switching then you can have those centrally (ie your HQ)
You cannot define SVI on WLC, it should be on a L3 switch in HQ. If you define DHCP on WLC then you need to configure "ip helper address x.x.x.x" on the L3 switch SVI interface pointing to WLC management IP address.
In fact I defined an interface on the WLC in vlan XX and defined the WLC as DCHP server and the created a WLAN associated to that interface.The Flex AP has correctly joigned the WLC. The same WLAN boadcasted to other Fex APs works fine. The only difference between those APs and the AP whose clients cannot receive DHCP release, is that this AP is behind a router which nats local IPs to a public IP.