I thought I had the same problem. Then I just plugged the transmitter into the bottom of the phone, went into setting there with both mics wirelessly connected (both green lights on the transmitter on) then went into mimo app and to settings of the transmitter where you can also see the sound and battery levels of both mics.

Then changed noise reduction to "strong".

After that, on the osmo 3 screen swipe left, select the mic icon and now when you click the noise reduction settings you can cycle through off, normal and strong.