Hi Donal,
for -5GD, please connect antennas to A and C port. Please not, that connecting 2x antennas will affect the performance. Also, we do not recommend using 2x antennas on the -5GD as there is no special mode to enable 2x antenna mode as on -5GH. As we checked with the modem vendor the -5GD should handle 2x antenna mode automatically, but you never know how good is the modem FW to handle it.
As for the other ports I would leave them unconnected as the received signal difference will be two large anyways.