Why is my wifi stuck in the off position

hi, i've had my iphone 4s for just about a year now and until around a week or two ago the wifi button was "greyed" out. i tried everything worth troubleshooting. i restarted, rebooted, reconnected, and reset everything i could. so i finally called apple and with my apple care plan they shipped me a new phone for free. when it finally arrived i had my recent iphone backed up to the minute so nothing would get messed up. i replaced my SIM card and everything was great! until about three hours later when THE SAME THING HAPPENED. a brand new phone and within hours of activation my wifi button was messed up again. i know that it isn't my actual internet because there are two other iphones in my house that have none of this problem. help me please!!