Hi there Rosie2014,
The best way to rectify or solve your issue would be to successfully get in touch with Optus. When you say messaging/chatting via the App doesn't work, in what way? Do you get an error message? Do you have the latest version of the My Optus app installed? Have you tried deleting and reinstalling the app?
Another way is to understand where your data is going. Have you looked at your data breakdown is the app or by logging into My Account? Have you checked your Apps information in your phone to see what ones are using data?
Overall if your mobile isn't connected to the home Wifi then that's clearly a major reason also. You've said all your other devices connect fine so it must be something to do with the mobile itself. Similar to the app questions, do you get an error message? Do you have the latest phone software installed?
Once you are able to get in contact with Optus, it's highly likely all these questions will be asked to try and help resolve your enquiry.