Default S4 mini sgh-257 weird issue

I have a weird issue here with my sisters phone, with no simcard in it gets a full signal status, as soon as I put in her simcard, or even my simcard it gets zero signal with random blips of a signal status then back to the zero signal symbol. Any idea what the issue could be? Is it a shot sim card reader ?

I tried a reflash & a clear cache but no different results

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