USB modems are partially configured in the CLI.
To enable a 4G modem, set
config sys global
set use-usb-wan enableand then of course
config sys modem
...to configure the modem.
Please be aware that this is not a 100 % guaranteed-to-work option. For testing, I'd get a prepaid SIM with some data volume and only after successfully using the 4G-wan port I'd buy a data SIM contract. For continued usage better avoid contracts based on data volume.
Usage is simple once you've got the 4G-wan port running: put the current wan port and the modem port into a "WAN load balancing port" (in v5.2) or configure ECMP on both, setting identical default routes but a lower priority on the modem port. This way, you can use the virtual port in policies regardless of the actually used wan port.
"Kernel panic: Aiee, killing interrupt handler!"