Using Wifi and LTE uplink in parallel


on my mobile moving travel OpenWrt I'm running the travelmate package for Wifi uplinks . For areas without Wifi I have another LTE network device via usb using kmod-usb-net-cdc-ether which just works like another ethernet device.

As the mobile LTE connection is going to be payed, I would like to favour available Wifis.
Therefor I see a feature request for Travelmate to add an option to have a fallback device that is enabled if no Wifi is available and disabled if a Wifi is available. I filed a feature request for this:

Having travelmate disabled for now, I'm not sure what happens, if both Wifi and the LTE device are enabled. Starting the LTE device as 2nd it seems it overwrites the default routes? I mean, it just works, having the device enabled it's going to be used as uplink. But is it possible to have a fallback default route so that LTE is only used if Wifi is not available?