https://synaptica.info/2020/06/26/ubuntu-20-04-rc-local-start-a-process-after-boot/

mcedit  /etc/rc.local

#!/bin/bash

exit 0

chmod +x /etc/rc.local

mcedit /etc/systemd/system/rc-local.service

[Unit]
Description=Local Startup Script

[Service]
Type=simple
ExecStart=/etc/rc.local

[Install]
WantedBy=multi-user.target

chmod 644 /etc/systemd/system/rc-local.service
systemctl enable rc-local.service
systemctl start rc-local.service
systemctl status rc-local.service