Hummm, I have a syncthing instance in a docker compose, so yeah I can access it through my ssl domain (syncthing.home.lab) but traefik takes care of everything.
Now if it’s on your local machine you’re trying to use your SSL certificate I don’t know, I always access it through the local ip (127.0.0.1:8384).
If I had to guess or give it a try, I would point the IP to my dns through my host file on my machine. But that’s just a wild guess :/
I think syncthing has a good documentation about it :)