I myself dealt with this issues for hours on end!! I googled and googled and googled away! Unfortunately these searches left me utterly disappointed and quite sad! Thinking there was no work around and that I would never get my mobile Authenticator to work, Something special happened, It finally worked!
So do like everyone says delete the app, reinstall, clear the data/cache! DO NOT OPEN IT!
Go to blizzards Account Overview page, Now on the right side of the screen under Security Check Up LINK YOUR PHONE FIRST!
Once I did this i opened the app and boom it all went soooo smoth!