You can connect the ANT-27 for either frequency, ANT-27 is a dual band sector. On the MR74, 2 ports are for the 2x2:2 2.4GHz radio and the other 2 ports are for the 5GHz 2x2:2 radio. So just make sure you're plugging the ANT-27 into the 2 ports on the SAME side of the AP. Even though the ANT-27 is dual-band, you'll basically be using it as a band-specific sector antenna, because on the MR74, the antenna ports are single band, two for 2.4 GHz and two for 5 GHz. So you would use 2 ANT-27 antennas per MR74 AP to provide the dual-band coverage.
