Hi,
we've got hands on an outdoor LTE-Router Teltonika RUT5xx.
the stock firmware is running a very ancient OpenWrt so we updated to LEDE.
The problem is, we can't get the LTE-Modem running, it isn't detected.
It seems the modem is a Huawei ME909u-521 or very similar.
the stock firmware seems to use the package "kmod-usb-rt305x-dwc_otg" but i can't find this package in openwrt CC or LEDE 17.01.
I'm hoping someone has an idea on how to make progress with this.
Regards
Install kmod-usb-dwc2
oh wow. i definitely thought i tried this one...
now it shows up in "lsusb":
Bus 001 Device 002: ID 12d1:1573 Huawei Technologies Co., Ltd.
what's next? tried some ways again but no matter what i install it doesn't show up anywhere in LuCI
what i did install so far:
kmod-usb-dwc2
usbutils
usb-modeswitch
luci-proto-3g
and afterwards i tried the QMI and the NCM variants:
kmod-mii kmod-usb-net kmod-usb-wdm kmod-usb-net-qmi-wwan uqmi luci-proto-qmi
comgt-ncm kmod-usb-net-huawei-cdc-ncm
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)