I'm trying to set up a particular network topology I haven't worked with before, and I'm having some trouble figuring out what I actually need.
In a traditional SOHO network, you would have a router that connects through a modem to your ISP (usually via a dedicated WAN port in the router), and your devices would connect to the router through ethernet or wifi. You could change your ISP and your devices wouldn't care, they're still connected to the same local network.
In the setup I have in mind, I would like the local network to work the same, but the "modem" should connect to a separate external wifi network that provides internet access. To give an example use-case, imagine you have a number of devices in your car, and you don't want to bother entering wifi details every time you go to a new location that offers free wifi. So you set up a router in your car, your devices connect to it, then drive to Starbucks and "somehow" (this is the bit I'm having trouble with) you tell the router to use the Starbucks wifi to gain internet access. Then you drive to McDonald's, and tell the router to use that wifi instead. And so on. So you only enter the new wifi details once, and all your devices immediately gain access to the internet.
I've been reading about APs, repeaters, and wifi bridges, but they don't seem to be quite what I have in mind. They all require that the local wifi share the same SSID as the wifi you are connecting to, essentially making all your devices belong to the external network (meaning you need to add the new details to each device). I want my local network to be separate, and the devices to be ignorant of whatever wifi I'm using to gain internet access.
So the question is, what kind of equipment and configuration do I need to achieve this?
I would also like to be able to extend this to other kinds of "internet providers", such as a cellular modem, satellite connection, or even wired connection, with priorities such as "use wifi if available, else cellular, else satellite", or all at the same time for added bandwidth, etc.
Below is an example diagram, showing in the orange box what I don't know:
Imagine the whole setup being mobile, and the external wifi changing over time.
