To my knowledge, you can only get an Open NAT on multiple devices that need to use the same port(s) at the same time by using UPnP, and even then it can depend on how good the router's UPnP implementation is for this to work.
I've got a SH2, but have only ever used it in modem mode, so can't comment on its UPnP capabilities, and have used various Asus routers instead, which have all handled multiple consoles with Open NAT via UPnP.
Also in Modem Mode, my ASUS router handles all my uPNP, and works great...
-- Red (VM SH2 AC Beta tester) Running on 1Gbps VM service with SH4 in Modem Mode, with a ASUS RT-AC5300 router. With this setup I get Fantastic WIRED and WIRELESS Signals in my home. I dont work for VM. But I work in IT, and I know my tech. My advice is at your own risk. If you are happy with my answers please press Kudo ?