Roon arc can't access Core

[Rugby (aka Daniel or Big Dan)]

Looking I found this

What is Carrier-grade NAT (CGNAT)? | Belong

Indicates that you are behind a cgant. See this thread about a fellownuser on Telestra

ARC and Telstra Smart Modem Gen 2

It looks like you might be getting a CGNat connection from your ISP. I would double check with them if that is the case. And, if so, see if you can get a static ip.