From my SMC network I can ping anything on the network, including everything on the 891 network (192.168.150.0 and 10.10.10.0 respectively. That is with using the static route on the SMC to tell everything on its network where to send things for 10.10.10.0. And it works. From the 10.10.10.0 network, I can ping 10.10.10.1 (the 891 on the one side), 192.168.150.150 (the 891 on the SMC side) and 192.168.150.1 (the SMC). I cannot ping anything else. If I add the route to my machine on the SMC network, I can then also ping that machine from the 10.10.10.0 network machine. But I still can't get to the internet. That is stopped at the SMC, I'm guessing.
I need the 891 network to be able to get out to the internet. It knows where the gateway of last resort is (it's a static on the 891, pointing to the SMC) but it just stops at the SMC (remember, I can ping the SMC with no problem from the 891 network machines.)