Hi
I am an old timer....new to systemd
I have no /etc/rc.local
It appears this post a bit old I admit UM did have it in the past
- It appears we may be able to create and use it such as by info in this link
https://www.claudiokuenzler.com/blog/783/ubuntu-18.04-rc.local-file-not-exist-launch-with-systemd-rc-local
eg from link
sudo echo '#!/bin/bash
some command
exit 0 ' > /etc/rc.local
make it executable
sudo chmod 755 /etc/rc.local
create a systemd service for it
sudo systemctl restart rc-local
check it is enabled etc
sudo systemctl status rc-local