My advice, having moved from Telstra myself.
Don’t buy the modem from new ISP / RSP, instead keep the Telstra modem. The Telstra modem works well as a modem only.
You can then buy a seperate Wifi system. If you are experiencing Wifi signal issues, I would suggest buying any of the available Wifi mesh systems available these days.
We bought a Netgear Orbi system a few years ago which has parental controls built in (Disney Circle). It works well and is easy to configure via the mobile app.
Wifi Extenders. These extend the Wifi signal, however they will slow down the network considerably.
Powerline Adapters. These can work in some situations but it depends on your home wiring.