How to activate my iPhone 6?

That bothers me for days.

I bought a used iPhone 6 on the net which was normally used by the seller. When I received it it gave a welcome page. I followed to activate it, but it continued to suggest that it cannot reach to the activation server.

That's the problem: My SIM card is in it, but iTunes says sorry, and cellular connection is available but just cant help, using WLAN doesn't work either. Activation fails forever... It's really upsetting.

So what's wrong with my phone, or the activation server(it ought to be alright), can somebody help me?