One of the big problems in systemd for many simple use cases is the
claimed obsolescence of rc.local, yet many times there is a real
need to just run some simple shell commands at startup. So, how do
you do it in the modern world? You write an additional systemd unit
file that will call your own rc.local script, or in the case of a
single command, call your command directly.
Place in /etc/systemd/system/rclocal.service:
[Unit]
Description=Run rc.local shell commands
[Service]
ExecStart=/usr/local/bin/rclocal.sh
[Install]
WantedBy=multi-user.target
20191108/DuckDuckGo systemd run shell commands at boot
20191108/DuckDuckGo systemd run shell commands rc.local
20191108/https://www.redhat.com/sysadmin/replacing-rclocal-systemd