Systemd has no idea what "up" means for networking
More accurately, the network-online.target doesn't mean what a reasonable person would think it means. It actually means that the base network drivers have been loaded, not that all the network interfaces are properly configured. For most of us, this is useless.
If you are using NetworkManager (default for Fedora), then you can use the
NetworkManager-wait-online service instead, e.g.
After=NetworkManager-wait-online.service
This has always worked for me.
Less kludgy than using a sleep, but obviously it only works if you are running NetworkManager and this service is enabled.