How to tell modemmanager eth3 (rndis) and ttyUSB3 is a modem

[AndrewZ]

Fibocom FM350-GL Support

I don't think ModemManager supports this modem in the given configuration (USB+RNDIS). From what I see this modem needs to be connected to PCI bus first, then MM will be able to talk MBIM to it. What you can probably do right now is remove MM, manually initiate connection from a terminal and read IP address(es), then manually configure the interface with the IP obtained from the modem. If you're comfortable with scripting you can probably automate this procedure.