You indicated that you have a service over the nbn® FTTN, FTTC or FTTB network, multiple phone sockets, and your premises may be more than 10 years old. It’s possible that the cabling in your premises is impacting the performance of your service.
If you have multiple telephone wall sockets in your home, the modem/router should plug into the main socket and your land-line phone (if you have one) will typically plug into your modem/router. If you have additional telephone wall sockets around your home, these may significantly weaken the performance and stability of your service.
Some older homes can have time-worn wiring that may also impact service.