To display the device in landscape mode you can use the following line of code:
descap.setCapability("--main-frame-resizes-are-orientation-changes", "landscape");
Note: Ensure that descap is an instance of ChromeOptions() class.
descap
ChromeOptions()
According to Screen Orientation in Appium the following line should be all you need:
driver.rotate(org.openqa.selenium.ScreenOrientation.LANDSCAPE);
[[UIDevice currentDevice] setValue: [NSNumber numberWithInteger: UIInterfaceOrientationPortrait] forKey:@"orientation"];