The WiFi connection is an added feature of the modem/router, which we don't have any control. We can only recommend to change the location of the modem or change the WiFi settings to lessen the signal interference that may cause issues with your WiFi connection.
If you want a new modem/router, you may purchase it for $99.95 + $10 shipping fee. You have the option to purchase a third party VDSL modem/router that will work with your NBN service. However, the VoIP that is bundled will not work.