You need a load-balancer/ or a proxy-web-server in front of Plone.
The web-server or load-balancer listens on port 80 (or in case of secured connection on 443 with a certificate in place) and forwards the request-packages to one or more Plone application server worker instances.
I do not have experience with offers by Digital Ocean. If you need only one or a few workers, then cheapest is probably to install a web-server like Nginx on the machine running Plone or on a separate machine if more than one worker is planned. On high-traffic sites an external load-balancer (I think Digital Ocean offers this) could be handy.
I've since switched to a vanilla Ubuntu 22.04 and running from
a UnifiedInstaller 5.2.9 instance, it is serving from port 80, so
I'm rid of the :8080.
It looks like I need to learn about nginx for providing https service.