How to prioritize a 802.11ac over 802.11n?
When the SSID is shared, the access point is responsible for band steering.


  Dual band operation with Band Steering detects clients capable of 5
  GHz operation and steers them to that frequency which leaves the more
  crowded 2.4 GHz band available for legacy clients.
  &mdash; Source


Enable band steering in your WiFi controller.