In case, you have an officially supported openwrt-device, i.g. RUT955, you can flash openwrt as an alternative to Teltonikas stuff.
I have a large fleet of 955s, running official openwrt, for IoT, with LAN as primary uplink, WiFi as second prio, and LTE as third. With auto failover, of course.
Works like a charm.
Teltonikas stuff has the disadvantage, always to lag most recent kernels and packages.