You need to setup apache web server as a reverse proxy for your nodejs web app so you can access it using the domain name, this is an example apache site for you to try it

<VirtualHost *:80>
  ServerName example.com
  ServerAlias   www.example.com
  ProxyPass "/" "http://localhost:3000"
  ErrorLog ${APACHE_LOG_DIR}/nodejs_error.log
  CustomLog ${APACHE_LOG_DIR}/nodejs_access.log combined
</VirtualHost>

Create a file called example.com.conf in /etc/apache2/sites-available/ with the above content, make sure to enable the proxy module and the new site with these commands

sudo a2enmod proxy
sudo a2ensite example.com.conf
sudo systemctl restart apache2

Now you can access your app using example.com as domain name.

Hope this helps