Why my iphone 5s cannot activate after reset

Why my iphone 5s cannot activate after reset?

the message appear as follow

Unable to Activate

"Your iphone could not be activated because the activation server cannot be reached. Try connecting your iphone to itunes to activate it, or try again in a couple of minutes."

I tried activate using Itunes. Still cannot activate it.

that is my iphone 5s Serial Number : DN***FFW

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