Everyone else is just telling you to do things in a way that is different, and while they are correct (you should use a unit.d/systems script for this depending on your distro), I’m going to actually answer your question since I know sometimes you just need a quick and simple way.
Depending on your version of cron, it may support special statements instead of the * * * * * notation for time.
The one you want is @reboot. Replace all entries of the schedule syntax with that, including the @, and the command will be executed only once when the system boots up.
Use that to start a script that checks for network connectivity on a loop with a sleep statement. Break the loop when you have connectivity, then execute your command, and exit the script.
Don’t ignore the correct way though. You’re better off executing this as a systemd (or equivalent) script. It’s barely more effort, and has the benefit of some nice built in logging and integrations.