I got an iPhone5 whenever it was it came out, and I thought 3G was just a bit dodgy, then i realised it was only not working when it was showing an EE signal/carrier at the top. I'm pretty sure i looked up online what was happening and changed a setting of some sort on my phone and it started working fine, and still does.
My mum's now bought an iPhone 5C and has got the same problem as i had, and thrust the phone my way to get me to fix it. I can't find any sign of what I did, nor what i need to do, to make it work.
Yep, we're both on Virgin. My 3G is working fine at all times. However my mums only works when the carrier shows Virgin at the top. When it shows EE, it says 3G but when you try to use it, the phone thinks it has 3G and after a while says that there is no internet connection. I remember this was a problem on mine and I'm sure that I had to make some alteration to a setting to make the phone accept using the EE signal but that's all I can remember.