I'm having the same issue with my 640. Our family recently moved. We had and have ATT Uverse. There is a difference. Before our home network was typical DSL 2ghz,, in our new home we have fiber 100mb using the 5k range. My 2ghz is not enabled. Not sure that would have anything to do with it...
Anyway, when I power on my phone, OS Build 14393.67 it only finds my HP wireless printer. No networks, no neighbors.. nothing. I checked the settings and it is setup to be searchable, ie. my HP printer found it. My home PC found it. I tired to manually search for hidden networks on my phone but says it can't connect to the wi-fi network.
Problem solved, I went into my wifi settings, enabled 2ghz and it popped up on my phone. Guess it's needed.