The MR86 antenna ports, the two on the top and two on the bottom of the unit, are all dual-band connectors. That is they have both the 2.4GHz and 5GHz antennas connect by a single connector, with the four connectors providing the 4x4 MIMO that the MR86 offers.
As such the MA-ANT-21 isn’t supported - since it’s only a single band antenna - and so you can’t select it.You’ll need 2x MA-ANT-27 for the MR86, these are the dual-band sector antennas, one connects to the top two antenna ports, the other to the bottom two ports.