I'm trying to install an official OpenWRT image on a Teltonika router (RUTX12) running RutOS and Busybox v1.30.1.
I'd like to understand if there are any pitfalls here: has anyone done this before? The Router I am using has two active LTE modules but I imagine that OpenWRT does not really care about this as it is just another network interface.
Are you sure it is supported?
Which OpenWrt image are you trying to install and where did you download it from?
LTE modems are NOT just a network interface, it's a bit more involved than that and may be working only with special drivers or scripts that exist only in RutOS. If you want plug-and-play LTE modems on OpenWrt you need them to have ethernet ports and be fully independent devices with their own web interface.
Afaik it's not supported so I'm not sure what are you trying to install on this device.
RutOS is basically a customized OpenWrt 19.07.7 though https://wiki.teltonika-networks.com/view/RUTX12_Firmware_Downloads
So it might be better to just add package repositories or something if all you need is a specific package.
Given that Teltonika posts GPL sources for all their (OpenWrt-based) firmware, porting to this device should not be hard. Just extract the device-specific bits from their GPL dump and glue it into OpenWrt.
Supporting the LTE modems using
modemmanager) should not be too hard either, I'd be surprised to find anything but the usual Quectel modules there.