With the last two Hearthstone updates, on iPhone it is not possible to change the server/region that you are logging into. The ‘are you sure you want to switch servers?’ dialog appears behind the login/password fields, so it can’t be clicked on and okayed, and you can only log back into the server you were using. (I tested with an iPhone SE and an iPhone 5.)
This is pretty basic web UI stuff, possibly prompted by changes to the code around Facebook logins? But, as we’ve seen with Warcraft Reforged, good, responsive in-game web UI is not Blizzard’s strong point.
With the iPad 4 app crashing at the start of every game (other thread) Hearthstone is unusable on iOS for me at the moment. I can’t imagine what the experience on Android must be like…