Alright, did the whole setup and it was (almost) smooth sailing.
The real problem I ran into was that by default, this OpenWrt snapshot does not have the needed packages for connecting the 4G modem.
Orienting myself along this guide, I modified the snapshot firmware @frollic posted further by adding the two packages qmi-utils luci-proto-qmi, which seem to be necessary for setting up the modem.
Further following the guide (though I did delete the default WAN interface) I got it all up and running, reboot-persistently.
This is what my Interfaces page looks like:
Hope this helps. I am just happy to have gotten it all working so quickly, so thanks again, frollic, I might have given up without your advice.