In general, it is best to have both all antennas attached to the router.
However, to have a better understanding the purpose of each channel, the most important ones are most left and most right antenna connectors, as these are primary receive/transmit channels for all supported frequency bands. Signals from the middle two connectors are for MIMO and diversity purposes and in diversity context signals coming from the middle antennas can often be neglected, meaning that only two connectors, on the device's left and right, are enough to establish data connection.
For more details you can refer to this thread.
Best regards,
