unable to activate iphone 11 pro max

From my experience, 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• Your version of iTunes is out of date. 
• 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.

See--> If you can't activate your iPhone - Apple Support