This is what you can do if the Wi-Fi-signal is weak:
Bring the heater into a room where the signal is strong and try and connect it to your phone there. Once connected, bring the heater to the room where you want it installed. Check that you have contact via the app to control the temperature.
PS! You can easily buy a Wi-Fi repeater that you can use to boost the signal into rooms with weak signal. Just make sure you connect to the repeater under the available networks at set up.