[Klojum]

Maybe I'm missing something

Maybe you're just overthinking things.

I've copied the same Advancedsettings.xml in the pastebin in each device but no media is shown on my second device.

"No media" is a bit vague to me. I see it as 3 separate things: creating a database, accessing a database locally, using the database with Kodi.
Perhaps that Failed to enable unit: Unit file connman-wait-for-network.service does not exist is a MariaDB thing, I just haven't encountered it before.

You could try to do:
systemctl disable systemd-networkd-wait-online.service
to disable the wait-online service to prevent the system from waiting on a network connection, and use
systemctl mask systemd-networkd-wait-online.service
to prevent the service from starting if requested by another service (the service is symlinked to /dev/null).

But again, I never have such network troubles with Ubuntu, including 20.04.
If you reboot your Ubuntu pc, and open a terminal and type mysql, what happens? It should get you into the mysql console.
You should also be able to log in with Kodi's credentials, using tools like MySQL Workbench or PHPMyAdmin.
Thirdly, there is the filling up of the Kodi database with data.

I've copied the same Advancedsettings.xml in the pastebin in each device but no media is shown on my second device.

So where exactly is the message "no media" coming from?
Also provide a full kodi.log the problematic device.