iphone 6 is unexpected Shutdown and Won't turn on after few days i try hard reset and after iphone is given error "unable to activate your Iphone because server is unreachable " this is problem can not be sloved even i install latest firmware on it

Here is Apple's support link-->If you can't activate your iPhone - Apple Support

And here are my supplementary notes:

Here are the reasons you might get the message that the Activation Server is unavailable:

• It really is unavailable. Rare, but it happens. You can check its status here: https://www.apple.com/support/systemstatus/
• Your version of iTunes is out of date. Currently 12.8 or later is required. 
• The phone does not have a valid SIM installed.
• Your antivirus or firewall is blocking access to gs.apple.com. Try temporarily disabling it.
• Your router's firewall is blocking access to gs.apple.com or to a port that it requires (443).
• Your phone is jailbroken, or it was at some time in the past.
• Your computer was used to hack or jailbreak some iOS device in the past (not necessarily the device with the current problem), and its network database was corrupted. Hacking includes using some questionable apps such as recboot, reiboot, Tinyumbrella, redsn0w, etc. If this is a possibility inspect your computer's hosts file. If there are any lines in the file that reference gs.apple.com that is the explanation. You will need to rebuild the hosts file.
• Your phone has a hardware problem. Take it to an Apple store for diagnosis.

From your description of installing the latest firmware I suspect the next to last or the bullet preceding it above.