I got it working with the DEBUG flag, but it starts working only after having the serial monitor started.
I made the following network settings:
const byte gateway[] = { 192, 168, 2, 254 };
const byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
const byte ip[] = { 192, 168, 2, 100 }; // Arduino has this IP
You have to use the same subnet (192.168.2.x) as your computer uses.
BR,
Chris