@Mark_Sealey, what is the gateway device AT&T supplied to you that is connected to your AirPort Extreme?
Also, Roon highly recommends not using an AirPort Extreme for a network router within Roon’s ecosystem for general performance issues (and from my perspective also for security, your version has not had any security updates for over four years, which may introduce some risks in your network). I am not sure if ARC will be impacted using an Extreme.
What @Jim_F is saying is that you most likely need to use port forwarding and the Nucleus likely won’t have the same LAN IP address as the iMac you are using for testing.