I recently did ride on Tempus Fugit which I ended intentionally at the 10 mile point. It was saved to Strava successfully, but hasn’t appeared in my Zwift activities. Something strange about this ride was that no one else appeared to be on the route during my ride.
Sounds like you had WiFi, Internet connection or ISP issues and that is why you didn’t see other riders and your ride didn’t upload to Zwift.
FYI, there is no way to manually upload activities to Zwift servers.
Strange that it uploaded to Strava. I know my Internet was weak, because I couldn’t get RGT to stay connected. However, there was no hint of trouble during the ride - except the solitude! Presumably I was in some sort of low signal/low graphics mode?