Solve One Plus One Wi-Fi Internet Connection Issue After Upgrading to Cyanogen 12

My One Plus One starts getting Wi-Fi Internet connection issue after upgrading to Cyanogen 12 (Android 5). Wi-Fi always shows connected. However, there is no Internet connection from time to time. Turn off and turn on Wi-Fi can fix the problem but after a few minutes, the Internet connection is lost again.

My router is pretty new ASUS RT-AC68U and support both 2.4GHz and 5GHz. It seems changing the Wi-Fi setting by turning off Scan always available and only connect to 2.4GHz  and solve this problem.

Here is how:

Go to Settings, select Wi-Fi, then open the menu at top right corner and select Advanced.

Turn off Scanning always available.

Select Wi-Fi frequency band (default is Automatic).

Choose 2.4GHz.