Replying based on your other thread about JTAG programming a custom board.
Tie MOD / AD_B0_08 to GND for programming and debugging. High is for boundary scan mode.
The easiest way to test would be to write a program in Arduino IDE and enable...
https://forum.pjrc.com/index.php?threads/teensy-4-1-ethernet-pins-socket.65288/#post-266193
Samtec MTMM-103-10-T-D-355
and
TE 2314820-6
Both can be sourced from DigiKey. These were the best I could find before Paul offered a kit.