In the meantime, I started up the App, and realized that it wasn't showing me the downstairs box. So then I had to figure out where in Settings Home Sharing was, get it turned on, and now I have a pseudo remote. It is not as satisfying, but it's a helluva lot better than not having anything at all.
ETA: The remote reappeared inexplicably on the dining room table the morning after I posted this. We're now pretty sure that T. was responsible. Altho we are happy it is back, we are not overjoyed about this.