MirrorLink in HK head unit not working

[RoadTrip]

Didn't see a more appropriate thread for this question, so here I am. The MirrorLink function on my 2017 Forester Touring Harman Kardon head unit is a case of close-but-no-cigar. Galaxy S10e smartphone pairs fine in general - but MirrorLink just doesn't want to work, no matter what I do according to instructions. Phone says the MirrorLink is connected when the USB cable is attached, but MirrorLink head unit app remains greyed out. The Samsung MirrorLink app is the latest version (1.1). I've done everything according to Hoyle, i.e. the manual. Anyone else have or had this problem?