AP and band selection is a decision making process made by the client device. Each devices' decision making process is unique and proprietary.
To help you better understand which AP and which band you device is connecting to, you can check the connection status of the client devices in Cloudtrax under: [Manage > Clients]
To make it easier to tell which AP the client is connecting to, you can enable the Use Access Point Name option under: [Configure > SSID X > Use Access Point Name]
Since release 6.2, you can also select which band and SSID can use. Setting the SSID to 2.4 or 5 GHz only will force the client to select the preferred band. [Configure > SSID X > Band: 2.4 GHz | 5 GHz]