Yes, Connect Mobile is designed for portrait mode only, and doesn't support rotating to landscape. However, there are a few screens in the app that support rotation, for that screen only.
Please fix this Garmin, or build a new app for the iPad Pro.
Has anyone gotten this piece of junk to work in a splt screen? I can get another app to run but the garmin app insists on centering itself. This is like some amateur app you find on github. You should be able to at least get 2 iphone screens on the screen; they could do that without having to re-write the application. They have all these products and they can't get their main app to run on a half screen or in landscape mode?
Garmin Connect is an iPhone app.
iPad may run iPhone apps since it was introduced, to let customers use their beloved apps waiting for the developers to build iPad native apps.
Apps running in this compatibility mode will all run vertical only and won't accept split screen, which requires iPad natives app. So basically, unless Garmin codes an iPad version you won't get anything that's been asked here.