We wouldn’t know, as it is not from us.
The linux version of syncthing is no package, just a compressed file. You just need to download the file, uncompress it and launch the syncthing binary. For your NAS the linux x64 would be the best option.
I installed Syncthing manually on a Synology for testing a while back: How to install Syncthing (arm release) on Synology? - #2 by wweich