Have you enabled new mobility on 5508 controller ?Both the controller are in same mobility group and there control and data path is up as well? Also check this document to configure mobility : http://www.cisco.com/c/en/us/td/docs/wireless/technology/...
This is what i understand regarding your setup5508 Anchor Controller --- 3850 Foreign Controller --- AP -- Client. You can run this command on controller to know why client is not connecting : debug client <MAC Address of Client>Also create a test WL...
You can set maximum power individually on each AP or you can set them globally to Power Level 1 which is the maximum power level by going to:Wireless > 802.11b/g/n or 802.11 a/n/ac > RRM > Power Level Assignment Method > Fixed 1