There is a possibility that the Mendix app is restricted to just the online profile as it is also strictly set to portrait mode (which I experienced earlier).What I did was connecting to headless local browser (make sure you machine port is open for external machines and using same wifi) by going first normally over the browser to your localhost and making a start screen icon ( "put on start screen"). And then you should be able to load your app, I expect also the offline profile.
May you can try that.
From what I know; currently that checkbox doesnt work, mendix havnt implemented that yet or not correctly yet.
What you can do to view your app local in offline mode is go to:
What you can also try is building a local app with the offline boolean on: You can read here how to do that;
After researching all combinations, the behaviour can be summarized as follows:
Both the offline and online profile for the same device (either phone or tablet) need to be available for the checkbox to appear.
Whenever the checkbox is used, the Mendix app still takes the online profile.
It seems that the checkbox itself is not functional?