The My Optus app will tell you, you can log into your account click on services and you can download a PDF copy of your bill. The PDF lists a daily breakdown of the total volume.
@ Have you tried logging into you account with a Web Browser?
Google how to change Primary DNS Server on your device and change the setting to 188.8.131.52 Cloudfare
Public DNS or 184.108.40.206 Google Public DNS and try again with the App or Browser.
If you have access to a device that is not on the Optus Network try logging into your account.