I’ve been able to do it on the app every time. The key is that you have to be on ship WiFi, though, not your phone data plan. But yes, you can do it on the app.
The first couple of times I got in line at one of the restaurants in case, but finished on the app while still waiting, so was able to ditch the line. The second time I showed others in line around me how to do it. That cut the length of the line down quite a bit once we all finished and left. 😊