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 

[Service]
Type=forking
TimeoutSec=0
RemainAfterExit=yes
ExecStart=/usr/local/sbin/StartupScript start

[Unit]
Description=rc.local replacement
CondidtionFileIsExecutable=/usr/local/sbin/StartupScript
After=network-online.target
Wants=network-online.target

[Install]
WantedBy=multi-user.target

create a script /usr/local/sbin/StartupScript with whatever binary or script you want to run on boot example:

#!/bin/bash
# this machine is on dhcp so I want to know what the ip address is when it boots
/usr/bin/ifconfig | mail -s "My Test Box IP" my@myemail.com
# I want to make sure wake on lan is turned up
/usr/sbin/ethtool -s enp3s0 wol g

make sure it set to be executed

chmod 755  /usr/local/sbin/StartupScript

now execute:

systemctl enable StartupService

fyi: In /usr/lib/systemd/system/StartupService.service the lines:

After=network-online.target

Wants=network-online.target

causes it to wait to start the targeted script until after the network is started

This was tested on Fedora 33