It can be difficult to correlate the 6 external antenna with the physical radios in the dual-radio CT521 systems. This document explains how to make the correlation.

When viewing the system upside down and from the rear, the 3 right-most antenna on the PCI faceplate are associated with the WiFi NIC located on the bottom of the unit. If you remove the four rubber feet, the bottom lid will swing open and you can identify the MAC address of the wifi NIC. This MAC will correspond to one wiphy entry in the LANforge GUI Port Manager tab.

It is not recommended that you open the top of the unit where the fan is located. This is where the 2nd wifi NIC is located which corresponds to the antennas shown in the picture below: