Q: My iphone 5s could not connect to internet and sustain wifi connection after upgrading iOS 8 . Is it same with iphone 5s ?

Hello Keyshow,

After reviewing your post, I have located an article that can help in this situation. It contains a number of troubleshooting steps and helpful advice concerning network issues:

iOS: Troubleshooting Wi-Fi networks and connections

Follow these steps to troubleshoot the above issues:

1. Be sure you're in range of your Wi-Fi router (access point).
2. Tap Settings > Wi-Fi and turn Wi-Fi off and on. If your Wi-Fi setting is dimmed, follow these steps.
3. Confirm that your Wi-Fi router and cable or DSL modem are connected to power, turned on, and connected to the Internet. If not, refer to your network administrator or Internet service provider (ISP) for assistance.
4. Restart your iOS device.
5. Tap Settings > Wi-Fi and locate the Wi-Fi network to which you're connected.
6. Tap  and Forget this Network.
7. Try to connect to your desired Wi-Fi network. 
   Note: You may need to enter your Wi-Fi password again if your network requires one.
8. Turn your Wi-Fi router off and on2. If your ISP also provides cable or phone service, check with them before attempting this step to avoid interruption of service.
9. Update your device to the latest version of software.
10. Update your Wi-Fi router to the latest firmware2. For AirPort Base Stations, install updates using the AirPort Utility.

Thank you for contributing to Apple Support Communities.

Cheers,
BobbyD