There used to be an app in the Play Store that allowed this. Can't recall what the name of it was....this was a couple years ago when I first got my S3. (I do know root was required) Hang in there and hopefully someone will have some solid input for you.
Enjoy your new Turbo!
1. Settings > Wireless & networks > More... > Mobile networks > Preferred network type > Select your network type
2. Open your dialer > type *#*#4636#*#* and select your preferred network type in there.
Please note, this resets after every reboot. I think you have to edit build.prop, which requires root, to force it in to any other network mode than LTE on boot.