As mentioned here previously @Marley16, an isolation test might be in order to establish the cause of the issue. Do you have a laptop or any other device you may be able to run a wired speed test on? Tech Support will run tests on your line and establish the nature of the connection, which I'm happy to do if you reach out with your account details in a private message. However based on the information here it may be a WiFi networking issue related specifically to the modem, not necessarily the connection to the property itself. Upgrading the plan to another speed tier would have increased the maximum speed threshold, which is a theoretical benchmark. However if you're having networking issues then it might be hard to get within that benchmark. Hopefully we can isolate the issue and improve the situation soon!
I unfortunately don't have any other devices which would be able to do a wired test. I have called Optus plenty of times and had technicians sent out which have tested on their end and said everything seemed fine.
Im hoping to call and change my modem as a last resort otherwise there's nothing more I can do on my end 😞