Disconnection occurs because the Bluetooth signal strength is not sufficient. This is likely caused by an obstacle between the Perifit antenna (the white wire) and your smartphone.
First of all, make sure the white wire at the end of Perifit is oriented towards your smartphone. To minimize connection problems, it is best to put the white cable on the front of your body.
Moreover, your phone should be positioned on your belly so it will be even closer to the white cable.
Finally, your legs should not be an obstacle for the Bluetooth signal. While you use Perifit, lie down with your legs bent and spread about 20 cm.