Glad that the OP was able to find a manual workaround. However, I am curious if the following Syno DSM process was attempted, and if it failed:
(1) Open DSM Package Manager > Settings > General Tab, and select “Trust Level > Any Publisher”.
(2) Move to “Package Sources” tab, and add the Synocommunity repo:
http://packages.synocommunity.com. Click “OK”. In the “Certificates” tab, you should shortly see Synocommunity’s RSA Key.
(3) Close the Settings and return to the main Package Manager screen. Scroll down to “Community” packages, and select it. You should see a lengthy list of packages in the Synocommunity repo, including (hopefully) Syncthing. Click to install.
If these steps are successful, you will no longer require updates from Synocommunity. Your initial installation may be an older version … but you should be able to execute updates within the Syncthing executable.