You can use a wireless LTE modem to connect your FortiGate to your wireless provider's network and allow access to the internet. You can connect your wireless provider's LTE modem to the FortiGate's USB port or express card slot just as you would connect it to your PC, laptop, or tablet. You can also connect your wireless provider's LTE modem to your FortiGate using FortIExtender. Some FortiGates also include a built-in modem. In most cases the configuration of the modems is similar.
Modem support just increases the number of options you have for connecting your FortiGate to the Internet. Because of higher wireless network data costs you may not use a modem for the only internet connection for your FortiGate. Modems are often used as a backup internet connection to take over some traffic if your main (usually wired) internet connection goes down. You can also connect a modem to be able to remotely manage your FortiGate through the modem interface instead of using the wired internet connection.
In most cases, installing an LTE modem requires connecting the modem to your FortiGate's USB port and enabling modem support from the CLI. FortiOS will find the LTE modem and automatically create a new interface for it. The modem will start up and connect to your wireless service provider. Then you can optionally configure SD-WAN and create firewall policies to allow traffic to use the modem interface.
FortiOS also supports older PPP modems that require more configuration, including setting up the PPP configuration the modem needs to log into your wireless provider's network.