Hi there, @mpi. Sorry to hear you're unable to open or send MMS messages.
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
Please also check out Can't send/receive Messages: Mobile Phone troubleshooting for some steps to try to see if they fix your issue.
If you don't have any success with the above, can I please bother you to reach out to us by a private message via our Social Media pages (official Optus Facebook page or Twitter page) where we'll be able to obtain your details securely and assist you further?
Alternatively, you can reach us on a smart device via our Messaging service on the My Optus app 24/7.
Did we answer your question? Please mark it as a Accepted Solution and if you see something that was helpful to you or the rest of the community, giving a Kudos is only a click away.