How to set Samsung Galaxy Alpha to 3G?

I have a Samsung Galaxy Alpha, unlocked ATT phone, and in my area only the 3G network is reliable. 4G works fine but only if I leave town. Right now I can barely call or text in my area, much less access data, but my old phone had 3G and worked perfectly in this area so I'm hoping if I can switch to 3G it'll be.... functional.

I found a thread for Samsung phones that showed options to switch between LTE/WCDMA/GSM by going under Mobile Networks > Network Mode, however this option doesn't exist on my phone. I also saw someone explaining how to go into "testing mode" and change the settings, however when I change the settings in test mode I don't see an option to save them and they don't save when I leave the mode. Can someone explain what I might be doing wrong or suggest a different method of achieving this goal?