IPhone Xr

SIM Card is not being detected: Try a new SIM Reader. On iPhone XR/11/12 (Pro/Max), it is modular and can be replaced. Before assuming it’s motherboard issue, buy a new SIM reader. If this fails, your motherboard is likely defective (I don’t do component level board repair, so if you know what to do in this case unseats of replacing the board, please add it here).

Side note: Chinese iPhones use physical dual SIM. You can interchange these SIM readers between models to make dual physical SIM work on any iPhone XR/11/12 model.