How do Ring sensors communicate with base station when WiFi is down?

I understand that when your power goes out and your internet connection is broken, the Alarm base station will operate on battery back up and communicate with the monitoring service via a cellular connection. But how do the sensors themselves (i.e. door/window sensors, motion detectors, glass break monitors) communicate with the base station? They have to send a signal to the base station before the base station can rely that to your app of the monitoring service. How is that signal transmitted from the sensor to the base station? I assume that, under normal circumstance, they rely on your WiFi router to communicate with the base station. Is that not correct? If it is correct, and your WiFi router is down, how would your sensors alert the base station that they've detected an event? Does the base station have its own WiFi? Again, I'm not asking how the base station would notify the monitoring service and get an alert to your app. I understand the cellular backup would handle that -- but I'm pretty sure the base station doesn't communicate with the sensors via cellular. I'm asking how a door sensor that's triggered would transmit its message to the base station if there's no WiFi. Thanks.