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Replaced antennae twice and it goes out after about 15 minutes. Does anyone know why? Thank you in advance

What goes out after 15 minutes? You need to be more specific. Also what does it say under Settings > General > About > Network (Hopefully Sim not provisioned or Phone not allowed.) Also there’s plenty of Antenna ribbon flex cables inside the phone. Which one? The L shaped one is for 2.4 GHz and 5.0 GHz Wi-Fi + Bluetooth along with the small Wi-Fi antenna housing piece at the top left of the phone inside that has like 5 screws on it and two screws to top of housing side. The antenna cable that plugs near the battery connector I think is for 3G cellular signal, could be 4G as well but I may be wrong as I’m not sure.