Hi @bjg,
Thank you for your patience and I’m sorry you’ve endured a long wait for an official response on your inquiry.
Aussie Broadband, as @oneofmany mentioned, has implemented carrier-grade network address translation. Depending on your account tier and geographic location, Aussie Broadband may simply be able to disable this feature for your account, enabling port forwarding. Another user’s success story is here: ARC not connecting to core on Asus RT-AC86U router - #17 by Jezep
@Simon_Arnold3, your question is by no means unwarranted. For a quick answer: Plex uses their own servers to do the heavy-lifting in circumstances where users must circumvent CG-NAT. There are several reasons why Roon has chosen a different implementation so far with the release of ARC. I recommend posting in Roon Software Discussion if you’d like to invite some clarification on this matter.