Since you built your own hardware, it’s really hard to tell. Which schematic did you follow? The only version that supports ethernet is OS 3.2:
yes the firmware detects the specific hardware version. ENC28J60 uses SPI, which requires 4 GPIO pins. Only version 3.2 made the SPI GPIOS available for ENC28J60, all previous versions use some of the SPI pins for other purposes.