I use the Yolink LoRa system with a temperature sensor and speaker hub. It gives an audible alert and sends out email and SMS if the temperature gets above my setpoint, assuming internet is working. Sensor batteries last about 5 months. It's not the ideal solution because you're relying on 3rd party services, but it has worked reliability for me for the past year. Also, once you have the speaker hub, you can add additional functions. I use it for a driveway alarm, gunsafe door monitor, greenhouse temperature monitoring and to remotely control my truck's block heater.