Hi there @GPea, thanks for reaching out. I know it sounds obvious, but the first thing I would do if you haven't already is power cycle your mobile device. After trying that, I suggest turning Aeroplane mode on for a few minutes then switching it off again and testing your connection.
If you do have another mobile device, I suggest testing your SIM in the other device to see if the issue persists. This can sometimes isolate whether it is a SIM/network issue or there is a fault with your mobile.
Another thing I recommend checking is our Network Status page. You can go to that site and search for your location to see if there are any network issues in your area.
If you do require any further assistance, please send us a message via the My Optus App so that we can follow up on this for you.
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