The way a get FlexConnect to work is:
setup a DHCP reservation for the AP in my management scope.
setup the AP switch port in trunk mode with management vlan as native, allow all the different SSIDs vlan on the trunk.
configure the AP in FlexConnect mode.
created a FlexConnect group with WAN/VLAN mapping.
add the AP in the FlexConnect group.
then my clients in remote offices get IPs from the local DHCP server.