secure configuration of the firewall

That does not look right to me. You want something more like:

# sudo ufw status verbose
Logging: on (low)
Default: deny (incoming), allow (outgoing), disabled (routed)
New profiles: skip

To                         Action      From
--                         ------      ----
22                         ALLOW IN    Anywhere
80/tcp                     ALLOW IN    Anywhere
443/tcp                    ALLOW IN    Anywhere

Note the differences in the defaults. You are allowing all incoming connections. One of the main points in enabling firewalling is to block unwanted incoming connections. So by default, you want to deny all of them and then specific exceptions for the service that you want to be available to the outside. In a common web server setup, that would be HTTP on port 80, HTTPS on port 443, and 22 for SSH. Though many change the SSH port to a non-default one. Let’s run through that set up.

First, let’s temporarily disable the firewall in order to make sure we don’t accidently lock ourselves out:

sudo ufw disable

Now we will set up default rules allowing outgoing connections and denying incoming ones:

sudo ufw default deny incoming
sudo ufw default allow outgoing

Then open the ports which we want to have available:

sudo ufw allow 80/tcp # http
sudo ufw allow 443/tcp # https
sudo ufw allow 22/tcp # ssh, change if you're using a different port

Finally, after we have opened the needed ports we can re-enable the firewall:

sudo ufw enable