Recently, more and more users complained about activation problem after upgrading to iOS 11 or later.

If your iPhone can't be activated or an alert says the activation server is unavailable or the SIM card is unsupported, learn what to do.

Check these things first

• If your iPhone uses a SIM card, make sure that the SIM card is in your iPhone. Even without an active contract, your iPhone might need the SIM card to activate.

• If an error says, "No SIM" or "Invalid SIM"; 

1. Make sure that you have an active plan with your wireless carrier. 

2. Update your iPhone to the latest version of iOS.

3. Restart your iPhone.

4. Check for a carrier settings update. Go to Settings > General > About. If an update is available, you'll see a prompt to select OK or Update.

5. Remove your SIM card from the SIM card tray and then put the SIM card back. Make sure that the SIM tray closes completely and isn’t loose. If you use a SIM tray from a different model iPhone or from another phone manufacturer, it might not fit properly.

6. Try using another SIM card. If you don't have one, go to your carrier's retail store and ask them to test with another SIM card. You might need to replace your SIM card.

• If asked to sign in with an Apple ID during activation, learn about Activation lock.

Still need help? Please keep reading.

Follow these steps

1. Check the list on the System Status page. If the box by iOS Device Activation isn't green, try to activate your iPhone later.

2. If asked to enter your password as you activate your iPhone, enter the password. (A password prompt means that Activation Lock might be enabled in Find My iPhone.)

3. Restart your iPhone.

4. If you're using a cellular data connection, connect to a reliable Wi-Fi network instead. 

If you still can't activate, here's how to activate your iPhone through iTunes or 3uTools instead:

• Make sure that you have the latest version.

• Check that your computer has an Internet connection.

iTunes:

1. Open iTunes on your computer. With the cable that came with your iPhone, connect your iPhone to your computer.

2. Wait while iTunes detects and activates your iPhone: 

• If iTunes shows, "Set up as New" or "Restore from Backup," it means that iTunes activated your iPhone. 

• If an error in iTunes says that the SIM card isn't compatible, contact your carrier.

3uTools:

Go to Flash & JB > Pro Flash, and then click Activate on the right bar.

Get more help if all methods are useless.

To check for network or activation issues and see if you need to replace your SIM card, contact your carrier. 

Use recovery mode to restore your iPhone. 

If restoring your iPhone still doesn't work, contact Apple Support.

Source: Apple