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.
@combimaster, I see what you’re stating. It does list the MA-ANT-21 under ordering information, although doesn’t actually state that it’s compatible... I would suggest you bring this up with your local Meraki AM to see if they can assist you.
the ANT27 is 12dB gain on the 5Gig which is just less than the 13dB on the antenna you have. You can select that one and safely be within FCC regs. You may disable the 2.4 radio since you won't have a 2.4 antenna.