If the device remains stationary for 15 minutes (e.g sitting on a desk or stopping mid ride), it enters Idle mode. Idle mode is a battery saving mechanism that we have added, whereby your Fly12 turns off automatically after sensing no movement for 15 minutes.
You can turn this off by connecting your device to CycliqPlus app. Go into the Settings>Config settings>Idle Mode. There, you can toggle to either turn the Idle mode on or off.
Alternatively, you may have a look at the Fly12 Hardware Manual. It runs through the manual settings which can still be accessed without the use of a smartphone.