Can’t change region within the app. 23/9/10

I have an Amazon prime account in Japan and all my content is for Amazon Japan and the region is set as Japan. I don’t why, but when I download this app it keep trying to sign in to the Amazon American, although I in Japan. I can’t change the settings. Tried deleting the app and downloading it many time. Didn’t work. It’s a really difficult app to use.