Sounds like your app only built for iPhone and not iPad. So, you’re essentially running an iPhone app on the iPad at the moment.
The latest Mendix native template looks like it should build for both iPhone and iPad by default, so I’m not quite sure how yours got set incorrectly. XCode determines what devices to build for based on a setting in project.pbxproj. Here’s a link to it in the native template
Specifically, do a search for the keyword TARGETED_DEVICE_FAMILY. The value should be set to “1,2” as described in this SO question:
Hopefully that setting is just incorrect on your app template and it’s a quick fix. Let us know how you make out.