To supplement what HKMOD answer, I have to say that with respect to most MDM functionalities other than locate or remote-control, whether or not the device agent is in the foreground is not important.

You might see that the device icon shown on the web-console become off shortly after the device agent has gone to background, the device is still in control.   For example, If there is any change made to an iOS  profile deployed to the device,  the updated profile will be silently pushed onto the device almost instantaneously. 

As another example, ii you click  the update button, the latest device information (e.g. battery level, Wifi SSID & RSSI, etc.) will still be sent back to the server for display on the web console  even if the device icon is off.   The "last status update" time will show the exact time when such device information takes place.  

The above mentioned actions are silently pushed onto the device via Apple Push Notification Service (APNS), which verify that the requested actions are from a genuine MDM server registered with Apple.  So, don't get worried your iOS is not in control when the device icon is off.  Actually, many new MDM administrators mistakenly worry that the device has been out of control when they see that the device's "last connect time" reported is months or even years ago.  This time is actually the last time the iOS device agent gets to the foreground by the device end-user.  It is actually very common that the end-user never need to tap on the device-agent icon on his/her device to do anything for months or years.

Hello Kamil,

This is expected behavior  of iOS device agent, if the device agent goes in background mode it disconnects and to locate the device the device agent should be in foreground mode.