The documentation is a bit confusing here. The MR28 & MR78 only have 2 radios and not a dedicated 3rd radio like all other AP models. So, Air Marshal (WIDS/WIPS) works differently. More on this can be found HERE.
I've also let Marketing know about the discrepancy so we can get the MR family datasheet fixed (matching the MR28 & 78 datasheets).
Ryan / Meraki SE
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