Lollipop 5.1 doesn't automatically connect to wi-fi

After I upgraded my Nexus 5 to Lollipop 5.1, it does not automatically connect to wi-fi because Google is blocked in China and then the phone treated it as no network connection. I have tried the solution here: https://android.stackexchange.com/questions/100657/how-to-disable-captive-portal-detection-how-to-remove-exclamation-mark-on-wi-fi However, it only removed the exclamation mark besides the wi-fi signal symbol, but the phone still won't connect to that wi-fi automatically.