iPhone unable to connect to server to activate

Hi Kimsenghuy2508,

Thanks for using Apple Support Communities. Since it sounds like you're having some issues activating your iPhone, we'd recommend starting with the following guidance from the related article, If you can't activate your iPhone or iPad (Wi-Fi + Cellular) - Apple Support.

"Follow these steps 

1. Restart your iPhone or iPad.
2. If you're using a cellular-data connection, connect to a reliable Wi-Fi network instead.
3. If you get an error message on your iPhone or iPad that says the activation server is temporarily unavailable or cannot be reached when trying to activate, wait a few minutes, then repeat the previous steps.

If you still can't activate your iPhone or iPad, try again using your computer instead:

1. Make sure that you have the latest version of macOS or the latest version of iTunes.
2. Check that your computer has an internet connection.
3. With the cable that came with your iPhone or iPad, connect your iPhone or iPad to your computer. Locate your device on your computer.
4. Wait while your computer detects and activates your iPhone or iPad: 

• If an alert appears that says "Set up as New" or "Restore from Backup," it means your iPhone or iPad is activated.
• If you get an error message on your computer that says that the SIM card isn't compatible or is invalid, contact your carrier.
• If you get an error message that says the "activation information was invalid" or "activation information could not be obtained from the device," use recovery mode to restore your iPhone or iPad.

If you still can't activate your iPhone or iPad, contact Apple Support."

All the best.