Putting my experience out there so others can find it.
Given I've switched my (current) primary desktop machine to Ubuntu, from Windows 10, I've been working through various like-for-like transitions of apps between Windows versions and Linux versions. Some aren't available and some have hiccups, and other things are just plain faster, and feel more efficient, like using a web browser.
In this case, I went to install the Linux version of the Synology Chat client.
Started by downloading the client from Synology, located in the Downloads section, specific to my model number. (Presumably that's so you get the a copy of the app that is designed for the architecture/CPU of NAS.)
Opened a terminal window and did
sudo dkpg -i Synology\ Chat\ Client-1.2.3-0232.deb
Turns out it wanted to have gnome-screenshot and gir1.2-appindicator3-0.1 installed first.
Ran this first
sudo apt --fix-broken install
Then ran
sudo apt install gnome-screenshot
sudo apt install gir1.2-appindicator3-0.1
Then ran the same Synology Chat install again.
sudo dkpg -i Synology\ Chat\ Client-1.2.3-0232.deb
Installed correctly this time.
Went to the Show Apps button 
in the bottom left corner and found the Chat client. Looks like this:
It didn't start. So, lets go find out why. Google'd first.
There's some bug in the code that doesn't handle spaces correctly. The problem is the directory name "/opt/Synology Chat" has a space, so I have renamed it
sudo mv /opt/Synology\ Chat /opt/SynologyChat
There's a symlink that had to get fixed as well in order for Synology Chat to be able to be executed from terminal.
sudo rm /etc/alternatives/synochat
sudo ln -s /opt/SynologyChat/synochat /etc/alternatives/synochat
Next, the desktop link needs to be fixed. Edit "/usr/share/applications/synochat.desktop" and change the Exec path "/opt/Synology Chat/synochat" to "/opt/SynologyChat/synochat".
Lastly, i had to log out, and log back in for the desktop icon to change. Apparently it's cached when the desktop is loaded.
After logging back in, fires right up.
