Unlocked iphone 4s invalid sim issue

[infinnocente]

I just got my unlocked iphone 4s 5 days ago and it tells me some times invalid sim and the network just drops... Airplane mode doesnt fix it.. I have to restart the phone and it works normally sometimes for a few mins and sometimes for a day or 2... Anyone faced this issue or have a solution?? Maybe a sim swap?? But i checked in earlier posts for the i4 they said it didnt fix anyting for them and i didnt find a solution..

Any help??

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I Have the same Problem in my iphone 4s! i m from Argentina.....i use Movistar