Beside of my last post, firstly you need a update to v1.13.1 on one of your devices. Second, on Synology you must set the option “Ignore Permissions” in the folder.
Which package you are running on Synology. Give you the Syncthing user and/or group your permissions in the Synology DSM? Maybe use the documentation for that.
I actually found a youtube video which provides a step by step process for noobs like me. I leave it for future reference for others: https://www.youtube.com/watch?v=uhmi96PesXM