Since "CDMA Only" is an option, I'm not sure why you would want an app to do it for you. Where I work, however, we are in a 4G area but it switches back and forth a lot. That switching causes major delays. I'd rather know if there was a way to switch to "LTE Only" so it could recover faster from a bad signal.
P.S. We make space flight electronics here where I work, so there is plenty of interference. It isn't just a fringe zone.