My iPhone 4S won't let me on my wireless network - the WIFI on off button is grayed out and I have to do a complete "reset all settings" or sometimes a "reset network settings" will work to allow the WIFI button to be set to on and work but soon stops

iPhone 4s won't stay connected to WIFI and WIFI on / off button becomes grayed out. Sometimes "reset network connections" fixes it and sometimes I need to do a "reset all settings" to get connected and allow the on / off button to work again but soon it reverts back to a non connection and grayed out button. I have tried a hard reset and it is on the latest iOs version. Anybody else have this problem and found a solution? I have also gone through the router / modem setup posted here and all thise settings are fine and my wife's 4s is exactly the same as mine and it holds the connection fine? Help please?