I tested these instructions on a mangOH Red a while back and they seemed to work. I don’t imagine it will be much different on an FX-30.
echo 1 > /proc/sys/net/ipv4/ip_forward
this command opens up eth0 to accept all packets
iptables -A INPUT –i eth0 –j ACCEPT
provide NAT so that many devices connected to the Ethernet port can use the same single IP provided to the cellular data connection
iptables -t nat -A POSTROUTING -o rmnet0 -j MASQUERADE
Forward related packets arriving on rmnet0 to eth0
iptables -A FORWARD -i rmnet0 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
Forward all packets arriving from eth0 that are destined to rmnet0
iptables -A FORWARD -i eth0 -o rmnet0 -j ACCEPT
To really make this work nicely, you will probably want to run a DHCP server on eth0 rather than a DHCP client. You should look at dnsmasq for that. Try poking around
/etc/dnsmasq.d/ to figure that out.
If you will just be plugging a single legacy device directly into the ethernet port, then you could skip DHCP and just assign a static IP to your legacy device and eth0 of the fx30. Then you need to configure the routing table of your legacy device such that the IP of the fx30’s eth0 is your legacy device’s default route.