Problem:

You iPhone can connect to a Wi-Fi router.
The Wi-Fi router does have an Internet connection. Other devices can connect to the Internet using this Wi-Fi router.
However you iPhone cannot connect to Internet.

Verification:

1. Tap on Settings, then tap on Wi-Fi.
2. Tap on the Wi-Fi network name.
3. Scroll down to IPV4 ADDRESS section.
4. Ensure that Configure IP = Automatic, IP Address = 169.X1.X2.X3 (X1, X2, X3 can be any number), Subnet Mask = 255.255.0.0, and Router may be empty.

Solution:

1. Reboot your Router.
2. If the problem still persists then please reset your iPhone by tapping on Settings > General > Reset > Reset All Settings.