Disable Waiting for Activation Popup
I faced this problem before. You can try to use this trick:


Go to Settings ~> Message and Turn Off iMessage + Facetime.
Turn on the Airplane Mode.
Turn on Wi-Fi.
Turn on iMessage like the first step. The system will ask you for your Apple ID If it is not added yet.
Turn Off the Air Plane Mode.
iMessage should work properly now.


Source.