Phone randomly cannot recognize my Wifi network
I've found the problem. Both the router and the phone was working correctly. The problem is with the Wifi channels. According to Wikipedia's List of WLAN channels, some places only allow channels 1-11, some others 1-13. Devices will provide support accordingly. My router supports channels 1-13, while the phone (I guess) supports channels 1-11. So when the router uses channels 12 or 13, the phone cannot detect the network!

Solution?


Buy another router which only support channels 1-11.
Logon to your Routers config through your browser to find whether the router supports limiting the channels. Refer to your Routers user manual on how to do this
Change the channel settings on the router to be manual (i.e not automatic), and somewhere in the range 1-11 (preferably 1 or 6 or 11).