SUCCESS!!!
There was one last step on the ATT NAT/Gaming page that I needed to complete. After defining the custom “Roon” service, I still needed to add that newly-defined service on the main page.
ATT modem screen now looks like this:
Roon ARC Settings now looks like this:
Hopefully my trials and tribulations will be useful to someone else stumbling across this thread someday. Which reminds me of the following:
Thank you @Suedkiez, @Robert_F and @Maarten_Duits (and others) for all of your help along the way! Now I’ll be able to listen to my favorite music on my trip tomorrow!