Hello Im not sure if this will help but at your NBN box in the garage it should have 4 different ports where a cable is connected to one of them in the NBN box and to the wall socket in your study, the ports might be called uni D-1 or uni D-2 or uni D-3 or uni D-4. Make sure that the WAN on the modem is connected to the wall socket and then go back to your garage and try connecting the cable to one of the other ports until the internet is flashing on your modem. If this doesnt work it can also mean that the modem you have recieved has not been setup correctly and Telstra should be sending out their own techs to be looking into it and fixing it for you or at least give you phone support on config on the modem.
Another thing that comes to mind from reading about your issue is that Telstra hasnt setup up your account right, I would be talking to someone in the level 2 tech support or someone higher as I have experience that the people you talk to just by calling the number just do the basic troubleshoot or try and pass it on.
hopefully this helps and good luck
