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.
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