I had the same issue, printer showed up in the Uniz desktop app but then disappeared after activation. I didn’t understand at first what “select the higher printer” meant.
Actually in the case of an OL version of the printer, Uniz offers a 7-day trial upon activation that unlocks the limited resolution of the printer, which becomes on par with a Plus version.
So all you have to do for the printer to come back in the app is manually select Slash Plus UDP (or Slash Pro UDP if you have a Slash Pro OL) in the drop-down menu from the upper-right corner of the app.