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So I have an IPhone XR with 256GB and the WiFi and Bluetooth greyed-out recently. I don’t really know how but it just happened. Is it the WiFi and Bluetooth antenna or the whole motherboard? Any knowledge is greatly appreciated.

If it is greyed out it will have nothing to do with the antennas. The problem will be the Wi-Fi IC. You will need someone with experience in micro soldering to do the repair as this usually requires fixing broken pads under the IC and re-balling the IC or replacing it.