After finally having found some time to work on the device I have been able to get almost everything working. Last thing missing is the SD card interface, for now I do not have a clue how to get that working. However, to me that is of peripheral importance. I'm happy to report the core functions (LAN, WiFi, WWAN i.e. LTE) are working very satisfactorily.
The device came not with stock OpenWRT but a fork from the LEDE time, that the manufacturer maintained themselves. In that firmware, LuCI is severely crippled and provides only basic functions. Either the firmware image checking in OpenWRT has changed since, or Hnet implemented some proprietary checking. This leads to the standard ways of OpenWRT upgrade to fail, as the stock image is determined to be invalid. Hence both LuCI and the sysupgrade commandline tool refuse to upgrade. However, the latter can be made to bypass the checks by using the --force option ("Use the force, Luke" ). This makes it paramount to triple check that the image is the right one, and it's not corrupted (check the SHA256 checksum). The image can be transferred to the device using SCP or TFTP.
After reboot you can customise the device as usual with OpenWRT. LAN and WiFi work out of the box but may need tweaking according to the purpose you have in mind (on my device, LAN is going to become WAN).
LTE is of course a different matter. I installed kmod-usb-serial, kmod-usb-serial-option, kmod-usb-serial-wwan and minicom. These provide some basic ways of communicating with the device. Using minicom on ttyUSB2 (115200bps, 8N1), the device brand and make can be checked with the ATI command. In my case, a Quectel EC25 is present, but since this is a PCIe module, something else my be present if you purchase a C108 for yourself.
For the EC25, QMI seems to work. So I installed luci-proto-qmi and kmod-usb-net-cdc-ether. After that, in LuCI you can add another interface (e.g. call it WWAN) with the protocol "QMI Cellular". Further configuration is straightforward: Enter your providers's APN, the PIN to unlock the SIM (You did insert a SIM before, did you?), etc. Enjoy your LTE access.