LTE is no longer working on my iPhone SE, WHAT is going on???

So, a little while ago, I realized that my iPhone SE is no longer working with LTE Data. I brought it into Walmart (Because it is a straight talk phone) and the guy said "It is no longer 4G compatible". So I thought "well crap... they must have just dropped it out of nowhere" but I can't find any one else reporting this issue so I'm wondering if what that guy was saying was right. Either way, does anyone know why my service no longer works on my iPhone SE? any help is greatly appreciated!