I’m another Aussie Roon user, I just did a quick search on Origins NBN plans, unfortunately they use Carrier Grade NAT, which is probably the cause of ARC’s failure to work. Unfortunately Origin no longer offer the option to opt out of CG-NAT and they also no longer offer static IP addresses so I do not know what you can do to get ARC working, perhaps you can try the Tailscale method other users have set up.
I found this thread on the whirlpool forums: https://forums.whirlpool.net.au/archive/3yqmj0zw
If you scroll towards the end of the thread a user who needed to set up port forwarding could no do so because they were told by Origin support that you can no longer opt out of CG-NAT or pay for a static IP address.
You should be able to set up a manual port forwarding rule on the Telstra Smart Modem but I’m not sure if it will help due to the CG-NAT issue but I think you should at least try.
Maybe someone from support will be able to help you when you move further up the queue.
For now, if CG-NAT is implemented in an ISP’s network, and you cannot mitigate it by requesting a public static IP address from your ISP, then the only way to get ARC to work outside your home is a VPN solution such as Tailscale.
The Roon team is looking at options for ARC to work with ISP networks using CG-NAT, but that remains in evaluation right now for a possible implementation in a future version of ARC.