I still expect there is some connection failure with AC12. Did you use Simnet-NMEA2000 adaptor cable? With that connection should be right. If not you have to find out CANH, CANL and power pins and connect them right to the bus. Note also that you should not have termination resistors on both simnet and NMEA2000 bus. When you connect them directly together, it is like one bus and must have totally two termination resistors.