Accessing IoT Devices on a different subnet?

If it relies on discovery, not without a whole lot of hackery. You would need an mdns broadcast across the subnets.
But if you manually specify IP addresses of things it works just fine. Every switch, bulb, camera, etc in my house is on a separate subnet.

Well, your devices in 192.168.0.x don’t know how to route to that subnet. You need to set that up in your router.

I will say that adding in routers isn’t the appropriate way to setup a network. VLANs, Access Points, a proper switch and router are the way to go.