This isn’t really true. Even IF your router would fail catastrophically in the right way to expose your Server to the internet, or of it actually “ratted your traffic out” to the ISP and the ISP cared (which it does not), it’s not illegal to hist Jellyfin, or put media on it which you own (which is not discernible from just… Media being streamed).

Also your ISP has no part in your local network traffic.