Like many others I had the problem after updating to Windows 10, that I couldn’t use 3G with the X98 Air 3G, cause the ‘next’ button was grey after I entered my pin.
And after many tries and many fails, I found a solution for this problem. At last it’s quite simple.
If your tablet is off, turn it on and boot up Windows. (don’t log in)
If you’re already logged in, then log out.
On the Login screen click on the network symbol (right bottom), select the 3G network and enter your pin.
Wait until the 3G network is connected and then log in to Windows.
And that’s it 🙂 Hope it also works for you!