Not sure if anyone has experienced this issue:
https://code.google.com/p/android/issues/detail?id=188867
Basically I can connect my Android 5.x, iOS 9.x and Windows PC devicesto my 2860ac with 802.11x without issue. My Samsung S6 with Android 6.0 cannot authenticate.
It appears to be an issue with Google tightening the security requirements of RADIUS and TLS and not supporting older (insecure) versions of RADIUS. Read comments #29, #180 and #186 in the article.
Could DrayTek look to upgrade the internal RADIUS server software please?