got it working:
Posting for future hackers….
I bought a 4G LTE modem from the official list. I chose the ZTE MF823 because it appears as an ethernet device.
Both nix and windows would see it as a network adapter and bring the connection up straight away. But not freebsd.
After several attempts at firmware tweaks and whatnot, i stumbled upon this russian thread: https://forum.pfsense.org/index.php?topic=76363.0
I don't speak russki, but i saw this command in the thread:
"usbconfig -u 0 -a 3 set_config 1"
so I started digging a bit.
My device appeared like this: ugen1.2: <zte,incorporated>at usbus1
And in the end, this command did the trick: usbconfig -d 1.2 set_config 2
The ue0 adapter appeared:
ue0: Ethernet address: 36:4b:50:b7:ef:44
ue0: <usb ethernet="">on cdce0
cdce0: <cdc ethernet="" control="" model="" ecm="">on usbus1
ugen1.2: <zte,incorporated>at usbus1
I could then set the wan interface as ue0 with dhcp, and the interface came up.
It's been stable ever since, even across reboots.
Unlimited 40/20Mbps for $20 a month... I like it.
Imgur album with all the settings and speedtest.
http://imgur.com/a/TUa23</zte,incorporated></cdc></usb></zte,incorporated>