Why won't 3G work on my iphone 5?

When I have 3G I can't text sometimes and can't use the internet or anything but it works fine when I have LTE how can I fix this.

IPhone 5's don't use 3G, they use 4G or LTE. To my knowledge, 4G and LTE are the two data services that are available to you and it depends on your location, which you will have access to. LTE is faster, which is why it works better, however LTE is not available everywhere you go. Sorry if this was confusing, i'm only 16

Take it to an apple store and explain your problem they will replace your phone free of charge

I down loaded ios 8.0 to my iphone 5. Ever since now it won't work on 3g, it only goes on line when i'm around Wi-Fi. But when i'm out of Wi-Fi range it just says not available offline. What's p with that? My 3g don't kick in. It use to work before I downloaded the ios 8.0 can anyone help me? Straight talk can't figure it out.