Smartphones connected to the WiFi network will preference that over the phone Data allowance or even use both to speed up connections. Many people are then using their fixed service Data rather than phone data around their homes. Perhaps there has been a change in usage of such a device or one has recently been enabled on the network.
The Telstra Routers do show the connected devices in a ‘Network Map’ with all the attendant data eg MAC address. They, depending on the model of router, also show the data used by the device.
Another issue may be that updates are occurring and failing resulting in multiple retries, not knowing what data allowance is held this could also be a reason for finding the allowance quickly disappearing. If it is a Win 10 version update this could be around 3GB of data each try.
I’m happy to Teamviewer into the machine to check possibilities but @clubman may not wish this to happen.
