Connecting Fortigate and a 4G Router

[IronMan]

I have a Fortigate-400E and a 4G Router.

About the 4G Router

This router has an IP address of 192.168.x.x. To get internet access, all I have to do is plug in a LAN cable from the 4G Router to a PC. That simple.

About the Fortigate

This firewall has a 10.10.x.x IP address range.

My Question

How do I set the 4G Router to the firewall? Right now I have plugged in a LAN cable from the 4G router to firewall physical interface 10. What configurations do I make? Do I change the role of the interface to LAN or WAN? What else needs to be done? Create a VLAN? Once this part is done, I can create a policy, that part I is not a problem.

Could someone provide a step by step configuration guide? It is so simple without the firewall but I have no clue how to get it done thru the firewall.

[ebilcari]

The easiest solution is to double NAT, just set the port 10 to receive IP from DHCP and create a policy to NAT (use outgoing interface) all the FGT traffic to the 4G router.

The recommended solution is to enable static or dynamic routing on both FGT and 4G router, you can take a look at the example below that shows the steps:
https://docs.fortinet.com/document/fortigate/6.0.0/Handbook/177888/static-routing-example
P.S If it's misleading focus only on the GUI steps, the CLI configuration is the same as done by the GUI but mentioned if you prefer to do it via CLI

[IronMan]

So it's forwarding traffic to port 10 directly? No need to enable wwan?

The role of the port10 should be WAN or LAN?

[ebilcari]

The role is just for ease of configurations. Changing the role will not automatically do any Routing or NAT-ing. Default route and Firewall policies will handle it.

you can read about interface role here: https://docs.fortinet.com/document/fortigate/6.2.12/cookbook/574723/interface-settings

[IronMan · Accepted solution]

I've go this to work. Here's a step by step for newbies.

1. Plug in a LAN cable to the 4G router and connect it to an interface on the firewall.

2. Set the Role of that interface as LAN.

3. Set the IP/Netmask using the IP range of the 4G router, not you internal network IP range. Manual or DHCP is up to you. I selected Manual so that the interface IP is always the same.

4. Disable Receive/Transmit LLDP

5. Enable DHCP Server.

6. Set Default gateway to "Same as Interface IP"

7. Set DNS server to "Same as System DNS"

Next, create a Firewall Policy where the Outgoing Interface is the interface that you just configured above.