I am really sorry to ask but I do not know how to solve my broadband modem problem NixOs.
“networking.networkmanager.enableFccUnlock = true” in the configuration.nix file did not make the expected directory in symlink.
But I can enable the modem with this command
qmicli --device-open-proxy --device=/dev/cdc-wdm0 --dms-set-fcc-authentication
/TheStig