no 3G connexion

Hello there,

I can not connect to 3G, as soon as I turn the cellular data and 3G phone network bars begins to disappear until it will go into research networks.

it is an iphone 4 ios 7.1.1 non jailbroken not unlocked too.

I tried to put the sim card on another iphone 3G work properly.

I also did a reset iphone but nothing.

Thank you for help please.

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