How did you come to this conclusion? My Sierra modems are all USB interface. You see there are 3 flavors of MiniPCIe sockets.I guess that means 11.0-RELEASE has no support for USB modems out of the box..
ls /dev/cuaU*
ls: No match.
ugen0.3: <UPEK> at usbus0
info: [drm] Enabling RC6 states: RC6 on, RC6p off, RC6pp off
em0: link state changed to UP
ugen2.3: <HUAWEIMOBILE> at usbus2
umass0: <Mass Storage> on usbus2
umass0: SCSI over Bulk-Only; quirks = 0x0000
umass0:7:0: Attached to scbus7
cd0 at umass-sim0 bus 0 scbus7 target 0 lun 0
cd0: <HUAWEI Mass Storage 2.31> Removable CD-ROM SCSI-2 device
cd0: Serial Number 0123456789ABCDEF
cd0: 40.000MB/s transfers
cd0: Attempt to query device size failed: NOT READY, Medium not present
cd0: quirks=0x10<10_BYTE_ONLY>
da0 at umass-sim0 bus 0 scbus7 target 0 lun 1
da0: <HUAWEI TF CARD Storage 2.31> Removable Direct Access SCSI-2 device
da0: Serial Number 0123456789ABCDEF
da0: 40.000MB/s transfers
da0: Attempt to query device size failed: NOT READY, Medium not present
da0: quirks=0x2<NO_6_BYTE>
(cd0:umass-sim0:0:0:0): READ(10). CDB: 28 00 00 00 8d c1 00 00 04 00
(cd0:umass-sim0:0:0:0): CAM status: SCSI Status Error
(cd0:umass-sim0:0:0:0): SCSI status: Check Condition
You defiantly picked a tough one. My first instinct would be checkout your Mint installation and install modemmanager-gui. It has a whole host of information about your device and will help you. You may even be able to install putty and use ttyUSB0 to send the commands to disable SCSI-Drive as per those instructions from github. Once that is done recheck your VID and PID in Mint and return to FreeBSD. See if your usbconfig dump_device_desc is showing the same VID&PID as before. Compare the two OS. If the same then either add to u3g&usbdevs and recompile or deal with devd. With a recompile all you need to do is add PPP settings to rc.conf as the device is detected correctly. Not a jury rig.
You generally need to have an active data SIM installed to get this going. Ubuntu/Mint is good for checking this. All you need to do is edit your broadband connection and add APN. Get it working there first and figure out FreeBSD.
You may need to switch it to this VID and PID like the mailing list post.
idVendor = 0x12d1 idProduct = 0x14dc
Maybe you could switch it with Mint modemmanager(mm_cli) and then it will be detected with FreeBSD. No compile or devd.
What is your current VID & PID in FreeBSD?