No bars signal on iphone 4s o2?

It has no bars, it doesn't say no service, but it has 3g now and then, please help its driving me crazy i have tried another sim in the phone and that is the same

Added (1). I have no warranty or anything i have bought the phone second hand

It's the phone. If you have AppleCare+ take it to an Apple Store and they will run a diagnostic on it and if it comes up faulty give you a new one for free.

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