The command uses the following structure:
nrfutil 91 modem-firmware-upgrade [<options>] <args>
nrfutil 91 imei-read [<options>] <args>
nrfutil 91 imei-write [<options>] <args>
nrfutil 91 help [<options>] <args>
To see the complete list of subcommands and available options, run nrfutil 91 --help.
All of the subcommands require that the access port for the application core is open. If the device is protected, you can open it using nrfutil device recover.