rotanid
5
we managed to get it working today using the current lede-17.01 branch.
mandatory packages:
kmod-usb-dwc2
kmod-usb-serial-option
ppp
nice to have:
luci
luci-proto-3g
minicom
usbutils
with the openwrt-18.06 branch it doesn't work the same way.
the modem doesn't even show up in "lsusb" output 
also, normal reboot (soft reboot) doesnt initialize usb modem device, you have to powercycle
EDIT: maybe it has to do with: (found by @feckert)