If you could not succesfully pair the devices with gateway, you can open pairing mode on the device with following steps.
1- Take out the batteries
2- Install the batteries while pressing middle button
3- Click middle button once
You will now see wi-fi icon blinking on the device display and now you can do the pairing on the APP.
Now the APP will find the device and you can now finish the installation.
After installation is done and you see the main screen with temperature display on the device screen, take out the batteries and reinstall them and restart the device.