Okay, I think I see the problem.
The SMC, like most such devices I would guess, is only going to do NAT for traffic with a source IP that matches its 'interior' IP range (i.e. 10.10.10.* if I read your example right). If the 891 traffic hits the SMC with a source IP of 192.168.150.2, then whooops, it's not going to be NATted. (And no NAT means it's not going out)
I very much doubt the SMC, or any other 'consumer-grade' NAT device for that matter, can be configured to do something as fancy as what you'd need.
One 'solution', as ugly as it would be (and it would break other stuff): can the 891 do its own NAT? then the traffic would hit the SMC with the 891's 10.10.10.whatever IP, and the SMC would happily NAT it again and send it out)