How to unlock my iPhone 5s without iTunes
My Iphone5S is 2 plus years old and need to unlock it. I went through the hoops to unlock it by contacting ATT, obtained a permission code for unlocking it. My iTunes on my MacBook 3.1 running OSX Version 10.6.8, told me that it had backed up my Iphone. Then I got a message on my computer saying that it could not be backed up because I did not have the latest version of ITunes. Unfortunately, ITunes tells me "The Iphone "XXXX" cannot be used because it requires a newer version of iTunes. Go to www.itunes.com to download the latest version of iTunes. However, when I go there it says I have the latest version, 11.4. I've already lost virtually all of my data and still apparently have a locked phone per my conversation with ATT today. They told me I needed to attempt to have Apple resolve the issue. As a side note I see that ITunes 12.3.1 is available but when I try to open it after down loading it I'm told OSX 10.8.5 or later is required.
How can I unlock my Iphone 5s without going through the iTunes hoop since that option apparently isn't available?
iPhone 5s, iOS 9.1