Hi 246,
Welcome to CrowdSupport.
Sorry to hear about the experience you are having with your WiFi Network Extenders. These devices are both suitable for ADSL, Cable, NBN connections, bearing in mind that they will need to be re-registered for any new modem that is now connected at your home to be used with that network
Dependent on the device/model of the WiFi Extender(s) you have, you will be able to find the steps online if you have not retained the documentation included with the extender when purchased/received.
The WiFi Home Network Extenders that we recommend can be found at: https://www.telstra.com.au/connectedhome/enhancements/getwifi#features