For compatible reason, rc.local support is still supported by new version of Ubuntu. But it is disabled by default. This will guide you to enable running rc.local while starting system.

1. Create rc.local if it does not exist.

Run nano /etc/rc.local. If the file does not exist, place this code below as the default rc.local file.

#!/bin/bash

exit 0

Run chmod +x /etc/rc.local to give the executing permission.

2. Create systemd service file.

Run nano /etc/systemd/system/rc-local.service to create the systemd service file and paste this text below.

[Unit]
Description=/etc/rc.local Support
ConditionPathExists=/etc/rc.local

[Service]
ExecStart=/etc/rc.local start
TimeoutSec=0
StandardOutput=tty
RemainAfterExit=yes
SysVStartPriority=99

[Install]
WantedBy=multi-user.target

3. Config systemd using this bash command below.

systemctl enable rc-local

All done. The rc-local will be run while system starting.