The short answer is, you can't - and there's not really any point doing so.
It's a router, not a mind-reader, it just follows what you configured - if you unplug a cable, that just means there won't be any internet anymore, not that it magically reconfigures itself to use another interface as uplink.
I guess travelmate may have some support for that, though.
You would be far better off just buying an OpenWrt supported travel router and putting Travelmate on it (will cost in the region of ~$30 or less). Then you will have a tiny, truly portable, reliable, all in one device that just needs a usb power supply to work.
This will do everything you could want from a "Travel Router" and free up the expensive and hard to get Pi for the type of projects it was designed for.
+1000 to this. There is also the benefit of the fact that they are fully integrated devices from a physical standpoint... you don't need to have USB wifi adapters hanging off the main device, and things like antennas are either internal, or designed to fold neatly and reliably out of the way so that things are less likely to break.
If you really want to, it is possible to change the numbering of the wifi radios by editing the name in the wifi-device section of /etc/config/wireless. Do not stray too far from the format of 'radioX' so there won't be conflicts with other names.
Always plug a USB radio into exactly the same one of the 4 physical USB ports on the Pi, or it will be detected as a new radio and assigned a new name.
In order to use a third-party wifi as the link to the Internet, it is necessary to have that wifi STA alone in its own network (not a bridge), usually named wan (available by default) or wwan (created by the "Join this network" script in LuCI). The firewall zone for the wan must have masquerade enabled so that pakets are NATted as they leave the router.
I was thinking about the GL INET series of routers - except I would probably opt to getting a Beryl or Slate model. Still, that's about $100 here in Canada. I already have a Pi4 sitting around doing nothing. I also have a Pi3B+ doing nothing as well.
Everything has been moved off these Pi's and onto a docker instance running on my Synology NAS. Figured I'd try to get this all working on a Pi before I went with a GL Inet router.