Try the following steps in order:
1. Make sure the phone is connected to 2.4 GHz Wi-Fi, not 5 GHz
2. Confirm the device is powered on and showing the pairing screen ('Use Mosqitter app')
3. Reboot the device: hold the power button to turn it off, then turn it back on
4. Close and reopen the app, then try adding the device again
5. Check that Bluetooth is enabled on the phone (used during the initial pairing step)
6. Make sure the phone is within reasonable range of the device during setup
If none of the above work, contact the support team at support@mosqitter.com.