Subnet routers: local network access to tailnet?

I’m still trying to figure out why the Synology package isn’t behaving the way I expected subnet routers to work, but I have a subnet router setup on my Azure VNet and local network. It’s amazing to see any device on tailscale being able to connect to devices in both of these subnets with blazing speed. But I still can’t get devices on the subnet to reach tailnet devices or devices on the other subnet. Is that possible with tailscale or do I need to implement some separate site to site?