Unable to activate

After whole long year i switch on my old iphone 6, which happened very smoothly but after getting into manually setup my phone is constantly showing that it's unable to activate and to try again later. But many other tries it's still showing the same message. The message is as follow " your iPhone couldn't be activated because the activation server is temporarily unavailable. Try connecting your iPhone to iTunes to activate it, or try again in a few minutes."

Now the problem is that I'm a windows user and don't have iTunes.

What should i do???