Hi,
Based on the information you provided, you will need professional help to fix your iPhone 12 Pro Max. The motherboard is affected by physical damage. (this is not a software problem)
iPhone 12 Pro Max has a sandwich motherboard. The top board is responsible for UI, and the bottom board is for communications. If the top board does not detect the bottom board, your iPhone will report a cellular error. The top and bottom boards are joined together with a connecting layer. As the iPhone was run over by a car, most likely, the connections are broken/disconnected.
To confirm this problem, go to settings/general/about and check Wi-Fi, Bluetooth ID, and Modem firmware. If any of these are missing, then it's a hardware problem.
BR
Francis
CelMetro