Reaching activation server problem after erasing iphone 5s

My phone is iphone 5s. I wanted to sell it. So I signed out my apple id from it and erased it by tapping on erase settings and contents. After that the phone restarted and I chose country. Then I connected it with wifi and tapped next. Then after a few seconds it showed that "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". Then I tried with itunes. It shows that "your iphone could not be verified". I restored it with itunes. It still shows the same message. I restored it with 3utools. It still shows the same message. I removed my iphone from the icloud through www.icloud.com/find. It still shows the same. what shoud I do? Thanks in advance.