I have a solution for you. First of all, use the Ethernet library from my fork (https://github.com/kamtom480/Ethernet). Secondly, you need to connect some pins on the Ethernet shield. Connect pins 11 (MOSI), 12(MISO), 13(SCK) located on the ICSP Header with pins 11, 12, 13 located on the Digital Header (11 -> 11, 12 -> 12, 13 -> 13). Now you should be able to use Ethernet with Spresense board. If something does not work or is unclear then let me know.