I have found a workaround but thought I should report it as it might be a bug of help someone else. When I to open the GUI outside my LAN I do not see all the settings on the settings page. Additionally I cannot make changes to them when I click save. There is not error it just does not happen. If I am on the LAN and use the local IP address to access the GUI it works correctly just not outside. I do have port forwarding setup for port 8384 and I’m pretty sure it used to work in the past. My working around it to just use VPN and get directly on to my LAN and I then have access.
There is an Insecure Host Check setting in advanced settings which you might have to enable. It checks that your browser sends the same host header as the listen address (which doesn’t match if you are forwarding the port most likely)
That check only applies when the listen address is 127.0.0.1:... which it presumably isn’t if a port forward works. (It is a problem when Syncthing is running listening on localhost and on the same machine as a reverse HTTP proxy though.)
I suggest checking the browser Javascript console to see what errors are reported.
I am forwarding on my router. Forwarding external port 8384 to my Ubuntu box that is running Syncthing. I get the page with my folders, etc. Its when I open the Settings that I have the issue.