@GucciChang,

If it has been a while since you set the device up, go into your router's network settings and make sure that you did not set any reservations for the device or change any settings that could affect the setup.

If possible, try using an IoT network or Guest network where you can ensure that you are only using a 2.4GHz connection. You may also consider first setting the camera up indoors, where it will have a more reliable connection.

What phone are you using? Make sure that you do not have a VPN, Adblocker or Private Addressing enabled on your phone. You may consider trying the setup with another phone to see if anything changes. 

Lastly, What router are you connecting your device to?