of course, im gonna present two examples, one pretty easy, and one more advanced:
EASY:
This one is for a ZTE MF820D 4G LTE, which is " unsupported"
On this device i had to turn off the " USB flash" before plugging it in.
config system modem
set status enable
set auto-dial enable
set wireless-port 4 <------ The serial port which you can communicate with the modem, you need to find out which one you can use, you should be able to enter ATI and receive some information back.
set phone1 " *99#"
set extra-init1 " AT+CGDCONT=1,\" IP\" ,\" ONLINE.TELIA.SE\" " <---- Force the APN
end
config system 3g-modem custom <------ This tells the FortiGate what device its looking for
edit 0
set vendor " ZTE"
set model " MF820D 4G LTE"
set vendor-id 19d2
set product-id 0167
next
end
ADVANCED:
config system modem
set status enable
set auto-dial enable
set wireless-port 1 <------ The serial port which you can communicate with the modem, you need to find out which one you can use, you should be able to enter ATI and receive some information back.
set phone1 " *99#"
set extra-init1 " AT+CGDCONT=1,\" IP\" ,\" ONLINE.TELIA.SE\" " <---- Force the APN
end
config system 3g-modem custom
edit 0
set vendor " Huawei"
set model " E398"
set vendor-id 12d1
set product-id 1505
set init-str 55534243123456780000000000000011060000000000000000000000000000 <--- This message content is used when you dont turn off the USB-flash on the device, this tells the device your drivers are installed and you want the device to flip to communication. The string is different for each device.
next
after the E398 has switched mode:
config system 3g-modem custom
edit 0
set vendor " Huawei"
set model E398
set vendor-id 12d1
set product-id 120c
end
You also have an undocumented command to use to see your USB devices:
fnsysctl cat /proc/bus/usb/devices
Debugging:
diagnose debug application modemd 255
diagnose debug enable
diagnose sys modem ?
execute modem trigger/dial/hangup
For example, if your want to start an interactiv session with the device, you could type: diagnose sys modem com /dev/ttyusb0 <--- you might have to find out the right com-port, ttyusb0/ttyusb1 and so on, this is important when you set the " wireless-port"
Hope this helps you and others, i have spent a few hours on this, as you can see 
