@AdamJ It does not work in the absence of internet, which is a completely idiotic solution in this type of watches, also intended for mountains and other remote places.
@shanghai-on-sea probably stating the obvious. But if you upload the OS file to Kamoot or similar (ride with GPS, Strava etc) then download again this should at the elevation.
@babychai with this trick I have been able to sync with 4 or more watches at once. The filter is there to avoid activities being synced multiple times if you repair your watch or so.