My iPhone 5C keeps losing signal and saying "searching" and then I have zero bars at random times even inside my house, or "No Sim Card installed" then it starts working. And at the most random times (mostly when I'm out and about) it says "Activation Required" and I can't use my iPhone until I activate it. Where it says "Use Cellular Connection" I click that but it always say Cellular Data is not working. It's really annoying. This didn't start happening until I updated to iOS8. Please help.