It’s possible when the hub updated and rebooted, its IP address changed.
From the tools menu in the app, can you get to the hub by tapping the “registered hubs” or “find hubs” buttons (while you’re on the same local network as the hub)?
Thank you @marktheknife. The ip doesn't change (I have Unifi Network give it a fixed one based on MAC addr). However, I did click on the find hub button, see the hub, clicked it, and all is back to normal.
Never have had to do that before / not sure why the hub was forgotten with the update. Anyway, all is well now -- and thank you for taking the time to respond.