No dice.The problem is that the MOD does not set a default gateway for outbound traffic.
I tried something like this:
myDesktop/other device in LAN <--> LAN/WAN router <--WIFI--> dedicated MOD-Router/RPi <--USB--> MOD
1) MOD-Router/RPi public interface gets an IP in the LAN/WAN from the LAN/WAN router.
2) MOD-Router/RPi USB interface gets an IP from the MOD in the 192.168.51.0/24 subnet
3) LAN/WAN router has a static route to 192.168.51.0/24 via MOD-Router/RPi
4) MOD-Router/RPI is configure to route
echo 1 > /proc/sys/net/ipv4/ip_forward
So far so good. All devices from the LAN can reach the MOD, but only one way! The MOD won't send anything back.
Solution: figure out the IP that the MOD assigns to the MOD-Router/RPi (here 192.168.51.106)
ssh into the mod (from the RPi),
/sbin/route add default gw 192.168.51.106 and after that it works.
So the MOD needs to be reconfigured, in which case one might just as well re-configure it to be part of the LAN/WAN directly.
PS. one way to make it work is to change your LAN/WAN to also use 192.168.51.0/24 and exclude the range which the MOD uses 192.168.51.50,192.168.51.150 and reserve 192.168.51.1. -- but really I'd be a lot nice if one could configure persistent settings on the MOD which survive updates.