This is not really a modem issue you just need to check for the connected device list on your modem settings to find out whether any unknown devices are connected to your home network or not. Since there is no option on the modem settings or GUI to check or monitor the data usage for the devices connected to the home network you can only contact your service provider to monitor the data usage or you can use separate application to track the data usage.

Login to the modem GUI and on the Home screen left side menu select Connected Devices > Devices to check for the connected devices