According to a thread on the pfSense forum, FreeBSD does have an rndis driver with 4G modems and you can use it by
Does this sound right? I would try it but can't find this file.Extract the if_urndis.ko module from the ISO. Copy it to /boot/modules in pfSense and make the file executable.
Add the following line to /boot/loader.conf.local to load it at boot, create that file if you don't have one yet: