Why is the Wi-Fi on my iPhone always getting weak signal?

I recently swapped phones to an iPhone 6 from an iPhone 5, and i noticed that where i would usually get Wi-Fi around my house on my iPhone 5, it was no longer reaching my iPhone 6, i compared it tom brothers who also has an iPhone 6 and put them next to each other and he was getting 2 or 3 bars of Wi-Fi whereas i was only getting 1, i would also be right next to the Wi-Fi box and still be getting max 2 bars, this is super annoying because i can't get Wi-Fi in my bed anymore, bare in mind this is a second hand iPhone so it could be possible that the person before me dropped it or something like that and the Wi-Fi thing in the phone broke a bit, but i have no idea really, I'm just putting out a theory, it would e greal if someone could help me how to fix this.

Problems in the reception.

My iPhone 6 Wi-Fi is very weak unless I'm RIGHT next to the router. I've tried all the software fixes, and determined it's hardware.

Will simply replacing the antenna or the antenna cables fix it or is this a common issue where the chip on the mother board is bad?

If it's the antenna cable or module, which one is better to try first? I don't want to buy the wrong one.

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