Setup hostnames like

Currently I struggle with some web application that not work correct if you do not use https:

Currently I own a subdomain and have acces to the letsencrypt certificate there.

I have the Idea to give every devicea a name like

and on my (synology) I download the letsencrypt ssl (may as cron-job?) so I could access its dockercontainer with ssl.

Is that possible?
Is it possible without have acess to NAS over Internet?
Is there somewhere a tutorial how to do that?