[lime-users] Backup uplink for a Libremesh router?

Hi, I've got a USB 4G dongle and have successfully hooked it up to a router running libremesh using the RNDIS documentation from the OpenWRT documentation site. However once that router joins a mesh, the IPv4 address data vanishes (it is connected via LAN). However, I don;t actually need this node to be connected, I actually just need to have the main router that has a direct WAN connection to the internet, to *also* have a USB dongle, but use it as a fallback or failover should the main WAN connection go down. I've tinkered with Mwan3, but it just broke my internet all together (I think because it uses static routes). I was wondering if something like this was even possible or if anyone's done something similar with a secondary uplink? Thanks again! Martin