As far as I know you don’t need PPP-Configuration for internal LTE devices.
As long as the correct driver is in the kernel or IPFire (or was implemented from you by self-compiling an own instance) and IPFire recognizes your device in the interface-setup, you need “QMI” or “MBIM” (depends on the chipset on the WWAN-module) to connect and get RED configured the right way.
“MBIM” is not implemented in IPFire by default, but I wrote a complete isntruction on how to do this here, including a script that configures the RED-interface afterwards:
Correct me if I’m wrong, maybe I just misunderstood your issue.
Greetings
Alex