By default, syncthing's GUI is listening on localhost (127.0.0.1) only, meaning you coudl not access it from a browser that is not runnign on your Alt-F box, which is is probably the case.
One solution could be to create a SSH tunnel to port 8384. Another option, simpler and more flexible but less secure than a SSH tunnel. is described below.
Install the syncthing package via the package manager in Alt-F, then start syncthing using the command line
Upon starting it (as standard user, root is not recommended), a default configuration file is created in /home/$USER/.config/syncthing/config.xml
You should stop syncthing via CTRL-C and edit that configuration file to allow remote access to the web-based gui, by default it only listens to localhost. The change required is
into
Under the <gui/> node.
Important: please pay attention that by doing so, the GUI is basically open for any device that can access your NAS on this port..A login password should be created via the syncthing GUI. This is why it is less secure than the SSH tunnel variant
Start syncthing again via
Open a browser on your local computer that is able to access the NAS , for example
http://192.168.0.4:8384/ if 192.168.0.4 is your NAS' IP afdress
Ths syncthing GUI is now accessible. At the top you are offered to upgrade to the latest version. see another post to do that step without incidents / errors.