Using the Oracle Cloud VPS (Ubuntu 22.04) + Docker + Certbot.
My domain is: memos.brayozz.online
I can login to a root shell on my machine (yes or no, or I don't know): yes
The version of my client is (e.g. output of certbot --version or certbot-auto --version if you're using Certbot): 1.21.0
I have installed memos app using docker. This app is accessible through port 5230 (default one during the installation process). I've managed to install an LE SSL certificate using Certbot (memos.brayozz.online) and it works fine.
What I can't achieve is to have the SSL also when I access the app, so memos.brayozz.online:5230.
Is it even possible? Please aware, that I'm a rookie in managing VPS's servers and SSH commands, so it is possible that I've made a mistake somewhere.
Depends. Note that usually you can't combine plain HTTP with secure HTTPS on the same port. Most software just simply does not support that. Usually two separate ports are used.
Also, if you're directly connecting to a specific port, you're also (usually) directly connecting to that piece of software. So on HTTPS port 443 you've got Apache listening. And Apache has the certificate currently installed. But on port 5230 you have the "memos app" (whatever that may be) responding. So you should also install the certificate into that app.
Other commonly used scenarios is to use something called a "reverse proxy" (Apache can do that) so you're using application A (the reverse proxy) to connect to a different application B (e.g. the 'memos app').
Or, if the 'memos app' does not support TLS at all, you could use a separate application to do all the TLS termination in front of the 'memos app' if you don't want to use Apache as a reverse proxy.
We can help you but I'd want to advise (and ask) you to try to be more elaborative in your answers. For example, if you say "it still doesn't work", the first thing that springs into my mind is "what didn't work? what did you have before and what did you change? Why doesn't it work? Any errors? Incorrect behaviour?"
All those kind of things you could have thought of yourself and immediately provided in your post. By just answering "it still doesn't work" you're putting all the effort on the volunteers of this Community, while it, IMO, should be the other way around. You do not require specialised knowledge to answer basic, generic debugging questions as "what did you have before and how did you change it".
You should not expect that URL, directly on port 5230, to change. If you're connecting directly to 5230, Apache is not involved. And if Apache is not involved, but you're changing something only in Apache, than you shouldn't also expect anything to change on port 5230.