USB CAN adapters can show as follows on Linux.
A serial port appears. slcand can talk to it.
A can network device appears. e.g. can0, shown by ifconfig.
Set bitrate for NMEA2000 and bring up CAN interface:
ip link set can0 up type can bitrate 250000
It seems SignalK doesn't do this itself.
Can open SocketCAN interfaces, like can0. Select Analyze → Decode As, and in the last column of the table you can click to set it to J1939. This will decode as far as giving you PGNs. I didn't find an NMEA2000 decoder, but this at least tells you data is flowing.
CANable is a USB - CAN interface using an STM32F072. CANable firmware does slcan. The same device can be flashed with candleLight firmware which does Linux native / SocketCAN. I bought a device from Aliexpress which came with candleLight.