Sorry, I don't speak English, I use a translator.
Ubuntu 18.04 xfce on which I installed the complete Plasma kde.
Dolphin file manager network is empty, nothing.
Thunar file manager sees everything on the network.
I enclose a picture, the two pictures were taken at the same time.
Why can't Dolphin see its own share or network?
Manjaro is the same when I download a Thunar file manager for everything I see, my own share, nas, pc, laptop. Dolphin is nothing.
How to improve?
https://mega.nz/file/Uoxx1BRI#XmOT8hVl_v6bQ8lMZO7erEhTU66JTZMuqfOavkg9zRI

Have you tried Right Click and Edit properties see what's in there? Personally I hid that. It is annoying to say the least it doesn't seem to be anything useful I can see.

You do realise that the Plasma version you have on Ubuntu 18.04 is heavily outdated, ditto for the KDE apps. I suggest you upgrade to the newer 20.04 LTS version to see if this is reproducible.

I used Linux Manjaro for 2 years and Dolphin produced the same. Manjaro was the latest version always, up to date.
This is something of a Dolphin bug but I can't fix it unfortunately.
I don't think it's my fault alone.
Plasma:5.12.9
Frameworks:5.44.0
Qt:5.9.5
Kernel:4.15.0-117generic
x64

Well, Network Folders need to be added, of course, depending on the type of connection different type of folders need to be added. You should at least see the option "Add Network Folder":
https://imgur.com/gallery/dLML6VS