FYI. I now have 4 Telstra Smart modems. My connection via NBN HFC failed in descending periods from 3weeks to 24 hours to 16 hours. Went through the endless diagnostics via Dashboard and then with an array of polite, super helpful Techs who solved nothing, but each one sent sent me a new modem.
The first 2 failures at least gave me mobile backup, but the latest failure meant NO internet and NO phone. Tech answer was that I must have had a short circuit in the modem. A new one arrived today, less than 24 hrs from my conversation with Support.
But......wait for it! In order to at least have a phone connection I reinstalled Modem number 2 and decided to remove the 2 plugs at the bottom rear of the modem that the NBN guy had attached to the old ADSL plug. Voila! The modem immediately went online and full phone service was restored. Download is back to 49 mbps.
It would appear that there is something in the old ADSL connection that was interfering with the operation of the modem. I have my fingers crossed that it will keep working and I have a brand new, unopened modem waiting just in case.
Hope this might help others who have been through the same frame. DC