rc.local is a very convenient method of firing off a process when the system is booted. It is being phased out.
To replace it you can do the following:
create a file named /usr/lib/systemd/system/StartupService.service
replace /usr/local/sbin/StartupScript with whatever binary or script you want to run on boot
systemctl enable StartupService
causes it to wait to start the targeted script until after the network is started
This was tested on Fedora 33