Having a large fleet of RUT955s, running heavily customized "official" openwrt, my RUTs also have RS485-USB converters attached, for ModBus. However, cell modem is activated quite late during system startup, in /etc/rc.local. Which has the effect, that RS485-connection via converter shows up as /dev/ttyUSB0, and cell modem to be /dev/ttyUSB1-5, consistently. May be, something similar can be done using Teltonikas stuff.
Running lot of "private" applications on the RUTs, "official" openwrt much more flexible to use.