1. Mobile phone firmware problem.
2. wifi parts need to be repaired.
3. Router Settings are incorrect.
4, the phone wifi setting is wrong.
solution
1. Erase and restore all previous wifi Settings.
2. Turn on the wifi option.
3, there is a blue arrow next to many wireless networks, click the blue arrow and then click Ignore this network.
4. Return to the home screen to shut down and restart the phone.
5. After restart, enter the wifi setting page for wifi setting.
6. Reset the router.
7, go to the mobile phone repair point to check whether the mobile phone wifi parts are damaged.