I would recommend trying to connect it to iTunes. Ironically, the article I was going to suggest that you look at (If you can't activate your iPhone - Apple Support) returns a 404 error.
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My iPad keeps on saying could not be activated because the activation server is temporarily unavailable. Try connecting your iPad to iTunes to activate it, or try again in a few minutes. If problem persists, contact Apple Support at apple.com/support
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My iPad keeps on saying could not be activated because the activation server is temporarily unavailable. Try connecting your iPad to iTunes to activate it, or try again in a few minutes. If problem persists, contact Apple Support at apple.com/support