For anybody that cares, you can access a device on a secondary router when you have a double NAT. The primary router will assign an IP address for the secondary router. To access a device on the secondary router, route traffic to this address and port number of the device. Then use port forwarding in the secondary router. This is the same concept as routing remote traffic to your public IP address.
I don't own an OBi202 or a NAS, so I don't know if this applies to the above problem.