Why does my iPhone say Activation Required?
Activation issues relating to the carrier are covered here:


https://support.apple.com/en-us/HT201407


This is also a distinct situation than when the device knows full well the servers are not reachable:


Your iPhone could not be activated because the activation server is temporarily unavailable


That being said, the device activation can also fail or hang for four reasons that are ranked in rough order of likelihood:


Network you are connected to can't get to Apple's Servers
Apple's servers are overloaded and you can wait 15 minutes to an hour and get activated without needing Customer Support to assist
Apple's servers are up and running and your device record needs Customer Service to intercede and correct a typo or sold date
Your device needs to be erased and restored


The first can usually be remedied by trying another WiFi network or making sure the network you are on isn't blocking traffic to Apple's Servers with filtering or firewall. The second item can also happen when your network is overloaded or there is general internet connection or routing issues.