I had (have) the same issue as the original thread.
I tried the *#0011# trick to no avail. The version of phone that I have does not allow to switch from 2.4 to 5 MHz.
I went to settings/Developper options. At the bottom there is a topic called Limit background processes
When u click on that, you get some radial options. I set mine to No background processes from Standard limit and the signal bars immediately fillled right up.
I had the issue at home on my Netgear router and here at work where I sue a P.O.S. Mac as access point.
Im sure I will soon find a bunch of stuff that sued to work and dont work anymore, but WiFi is back!