One guess would be that it won't let you sign in using "join", that's to register as a new member. Clicking "Sign In" and then using your existing username and password will log you in.
The "this forum is not currently available" error may be an issue with the capacity of your mobile internet service provider's available data stream. I looked this error up on the app developer's site and they suggest that the app's cache may need to be cleared.
Here are some screen shots from the android version of the app.
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You should be able to browse the forum without logging in by clicking those 3 little bars in the upper left side of the screen to pop out this menu. Then click "Browse".
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