With the newer firmware versions for the GEM420 there is the option for IP Passthrough which will enable the GEM420 to work in modem mode and provide only the 3G/4G connection so it can be connected to the WAN port of your office router / firewall.
In this example we used a second router (Proroute H685) as our office router/firewall and connected the WAN port of the H685 to LAN port number 1 of the GEM420 and set the H685 to use the WAN port for it’s internet connection.
The H685 requested an IP address from the GEM420 and we could then access the internet via the WAN port of the H685 through the GEM420.
We then checked the GEM420 LAN Client list and could see the H685 connected and the MAC address of the H685 which we then copied.
Then in the GEM420 APN settings page, once we had configured the GEM420 APN settings so it could connect to the internet then we could activate the IP Passthrough and enter the MAC address. Once this is done the GEM420 will disable NAT and pass through all traffic to the device connected to LAN 1 of the GEM420 with the MAC address entered.