It may have something to do with the PLEX version you have, Plus, I can only read English, Sorry.
If the images shown are the NAS version of the PLEX app, I have to point out, mine was a lot different to what I see with yours, mine instantly showed me options to add my media.
I did install a DLNA server app which automatically added Video/Music folders to the server, where I placed my media,… inturn made the process smoother, as my media didn’t show up at first… IMHO.
I’m using a TerraMaster F5-221 so the process maybe a little different to what you are using,… Still,… You’d believe adding your media would be the simplest task.
I’m Sorry, I don’t believe I can be of anymore help to you, being relatively new to using NAS machines myself.
If it continues to elude you, I’d Uninstall and start again, believing I’d have missed something?
The problem is that I have no icon at this point to add libraries and it cannot find my server even though I have installed it on the NAS.
I reinstalled Plex, but I still can’t see my server.
Just a thought,… Is your NAS server software upto date?
Maybe it’s a Glitch with the particular PLEX app,… What is the PLEX version you have installed on your NAS?
Have you contacted the company who sells the NAS, they may have an FAQ on their website pointing to issues installing PLEX or others may have had the same issue and left a resolution.
The easiest way is to connect my laptop to the DMZ (in the same subnet) and connect the NAS to my Plex account. After that I can connect to my private network again.