@Greg_Basser, I believe Aussie Broadband uses CGNAT to conserve their IPv4 addresses. Roon ARC does not work over networks using CGNAT, but it may be workable if AB can assign you a static IP address:
I actually put a static IP in place when ARC was released and everything was working fine until recently
I have fiddled around turning various settings off then back on, redoing the manual port forwarding to the settings I previously had (deleted and replaced with the same 55000), rebooted my router and my server twice
Everything now works as it should
No idea what was the real fix but it worked about 5 minutes after my post was approved
My advice for future for others is to go thru all of the Roon support steps and reboot everything