It had automatically configured itself during the install. The ARC app would show my music but would never be able to play, always saying poor connection or core not found. I just logged into router and see that the port is set up and active. The port number matches the number on the core, in the settings. I made no configuration changes on the router, simply observed the settings on router and on core. Then I tried again from the ARC app and voíla it inexplicably is now working. For now. Doesn’t inspire confidences, so will see how it goes. Thanks for the article on the port forwarding.
I take that back. I think it was streaming something when it ‘worked’. When I try to play something from my library (a rip of a vinyl) that isn’t available on a stream service, I get core unreachable or poor connection, and it doesn’t play. I’m staring at Core and it is running and present. I have yet to get this ripped vinyl to play over ARC on LTE, though it plays on ARC with phone connected to wifi.