You should check the flex connectors are all seated properly. It does sound like a connection problem and the 11 Max is a layered board. The solder they use to connect the two layers in my standards is poor quality and the phone only needs a slight drop for the solder balls to crack and become oxidised.
I think you will have to have it looked at by a reputable repairer who would separate the board, check the pads than re-ball it with some quality solder.