This question already has an answer here:
I just purchased an iPhone 5s and attached to my iCloud. After that it seems that App Store applications were not installing properly. I did a factory reset and the following screen is shown:
Could Not Activate iPhone
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 few minutes.
If this problem persists, contact Apple Support at apple.com/uk/support