If you close the Sonos App fully (slide off screen) and cycle the iPhones Wifi network connection off/on, and reopen the App, does it then find the Port?
Not weird at all really. Spotify Connect uses a cloud based bridge between Spotify and Sonos. The Port is connected to the internet fine, but your phone simply can’t discover it on the local network. Have you tried (a) restarting the phone, (b) restarting the Port, or (c) restarting the router?
An hour later (and a trip out of the house) and the problem cleared itself. Thanks for the explanation of what was going on and the problem solving steps for next time. Much appreciated.