The SIM card needs to be in the device at the time of activation so it's possible this missed step may have caused you some grief.
Can you please try to perform a Network Reset on your phone. This will delete all saved network settings and WiFi passwords. This is to refresh all your Network settings on your phone without losing any data.
To do this, please follow the steps below:
iPhone - Tap Settings > General > Reset > Reset Network Settings
Samsung - Settings > General Management > Reset > Reset Network Settings
Also, try the troubleshooting steps on this guide → https://www.optus.com.au/for-you/support/answer?id=20001 to see if they fix your issue.
If you're not having any luck, please reach out to us via our Social Media pages (Facebook or Twitter) where we'll be able to grab your details securely and take a closer look for you.
Alternatively, you can reach us here → app.optus.com.au/msg-us?tags=OCAYCMSG on a smart device via our Messaging service on the My Optus app 24/7 Please let me know if you have any further questions or need assistance with anything and I'll be happy to help.