Airport Express and Mobile Hotspot

No.. there are no drivers for the hub in either the modem nor in the Airport. Almost everything out there works on drivers loaded on a computer so the USB can talk to ethernet.. but it is the wrong way around for you.

These converters do not have onboard processor.. you would need a much more intelligent device like raspberry pi which would be able to take packets from ethernet and convert them to USB and visa versa. Even those projects are mostly about using the pi in the wrong way. It is difficult to find a suitable project where you can build a true USB to ethernet converter.

In fact I would either look for a different 4G device.. one with an actual ethernet port on it.. these are readily available .. just look up 4G wireless router. Also called LTE devices. A lot of routers nowadays can also plug into a USB 4G modem or tether to a phone. But if this is your main network device.. then buy the real deal as it will work much better than a cobbled together bunch of bits and pieces.

Good find.. buying the right tool makes a big difference.. making up a tool from disparate bits and pieces is generally not successful or reliable. The bit extra you spend pays back heaps in service.

Let's start with the easy part first and confirm that the USB port on the AirPort Express is there only to connect a compatible printer. No other services will work at the USB port on the Express, so no value in trying to connect anything but a compatible printer to the USB port on the Express.

As far as the AirPort Express being able to supply power to the Hub, the USB port on the Express will not charge an iPhone or iPad, so I doubt that it would supply enough current to power the Hub if that was what you are thinking.

If the USB Ethernet Hub really does the capability of converting a USB to standard Ethernet, then in theory you could connect the AirPort Express to the Hub that way.

Whether or not the Hub really does have the capability to accept a USB signal and convert that signal to standard Ethernet is open to question. I've tried things like this in the past, and none of them have worked, but......I suppose you never know unless you try.

Maybe if you could provide the make and model number of the Hub that you are looking at, we might know more about the possibilities, or another user might have already tried the same product for a similar purpose and could provide some first hand feedback.

If you have not yet purchased the Hub, but are considering doing so, it might be a good idea to understand the store's return policies before you buy. Even if someone says that things "should" work, that does not necessarily mean that they "will" work. In situations like this, you just have to try things out on your network to find out how well.....or if.....they will work.

As far as having the Express "join" a hotspot, I have never had any problems setting up an AirPort Express this way. The A1264 and A1392 versions of the Express join the network and provide an Ethernet signal if that is needed for other devices.

A good explanation for why things won't work. A simpler explanation might be that the Ethernet port on the Express is looking for an IP address, and USB does not provide IP addresses as far as I know.

Sometimes about this time in a discussion, another will post to say that he has somehow figured out how to make things work. We'll see.