My Optus nbn hfc speed keeps up and down, sometimes very slow. The Optus guys didn’t change anything. They always let you switch off and on. When you complain to nbn, they apologise for any inconvenience your connection issues may be causing. As this is a problem with your services you will have to contact your Retail Service Provider . So how we can do? Sadly bad luck . Waiting them to upgrade the hfc to fttb one day. No hope?
Sorry to hear of your issues, NBNCO are correct, all dealings need to be done with your retail provider.
Things that can affect your speeds are, but not limited to: Network Fault, Usage, Congestion, Wireless.
Have you spoken to anyone at Optus that has done anything other than just powercycle?
Have you tried any isolation testing yourself to rule out usage and other devices in your premises?
I would presume that since they are not raising a fault or sending a tech they are not seeing any issues on the actual network causing your issues.
Try some speedtests whe the connection is slow, then disconnect everything on the network and only connect 1 PC via ethernet to the modem and run another speedtest to see if it is any better, that will help rule out usage and other devices if the speed is still slow. If the speed is better you may just find its usage within you premises causing the perception of slow speeds.
All the best