GPX from Fenix3 not imported inside InReachMap

How many points in the track? There is an upper limit to the number of inflection points in a route. When you import a track into inreach.garmin.com, the software does a certain amount of "thinning" of the track to reduce it to an acceptable number of points. However, I have seen a few posts like yours which seem to indicate that the import fails silently under some circumstances. I'd suggest that you break the track up into smaller segments to see if you can import it in pieces.

My recollection, which is probably incorrect, is that the maximum number of points in a route is around 500. Because of the thinning, your track probably won't need to be that short. But it's worth a try with a shorter track to see what happens.

I don't know how the Fenix3 works, but if you have waypoints associated with the file, it will fail on the inReach map. If there are, you'll need to edit out the waypoints. DeLorme treated these as a separate file to maintain. I don't see any reason why this has changed under the Garmin branding yet.