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I opened your form and I can see that it is already mobile responsive. This means that the form will adjust automatically depending on the screen size. It will be responsive even if you rotate your device in portrait or landscape mode. We actually have a guide on How-to-make-forms-mobile-responsive.
If I understand you correctly, you would like to prevent the screen rotation of the device? If that is the case, then you need to configure that in your device settings and not in the form.
Hope that helps. Thank you.