Well, I've never configured l2tp so I don't know what this is suppose to work like. But I notice the l2tp address in both your configuration and the example is a /32 single host mask. So this means windows does not know about the rest of that /24 subnet from the raw information on the interface itself.
I'm thinking you need to install a route on the windows box to the remote nework to your l2tp interface. You'll need to google l2tp routing and your windows version to see how this is suppose to work.
Here is a thought if you want to just try and hack this together. The command below is the temporarystatic route until reboot command -p makes it permanent but in this case I think you would use a batch file to launch when connected and let it expire with the end of your windows session.
route add 192.168.21.0 mask 255.255.255.0 192.168.21.211