Thank you for reaching out!
Unfortunately, passthrough mode is only available on a single modem.
There would be a few solutions to achieve this:
- Using an RS cable for a complete modem passthrough;
- While this would be possible using a USB hub, it would mean, that the Fortigate device would need to control the modems and transfer the data using AT commands. This would also limit the bandwidth quite a bit.
- Using VLANs as mentioned before, but turning off NAT (only if the carrier can provide more than 1 IP address per SIM card);
- To avoid double-NATing, separate firewall zones can be configured for each SIM with Masquerading flags disabled for both of them. This will disable NAT on the RUTX12.
- In this case, however, SIM cards will need to have more than one public/private IP address, as the router will get its own IP address, as well as the FortiGate device.
I'd like to ask you for your topology, perhaps we'll be able to find another solution.