Hi - frustrating when things go in unresolved loops. Have you considered cutting your loses and looking at migrating to an NBN service. This site will show you which type of NBN service is available in your locality.
Understood. Some people use Mobile Broadband Hotspots or Modems as a fallback or temporary semi-permanent fix for their situation awaiting the arrival of NBN, but it depends on how far away you are from local cell towers and how strong the signals are . There are many options available subject to your needs and budget.
This URL will show you the carriers that are near to you, which frequencies they are using to broadcast, so if you pursue this option, when you look at equipment you will know if the device is compatible to your carrier of choice - the stated frequencies supported in the various equipment will be shown in their respective datasheets.