that’s a damn fine question.
you can get 4g routers that support it, and it shouldn’t have any hardware dependency, so it may be a limit on how the network stack is setup on the phone OS and how the hotspot stuff works.
I see a bunch of AT&T users complaining about issues with routing vs not having issues with Verizon/Sprint, so it’s possible there’s some network defined component at work; I’ve never looked much into the data connection settings that are pulled down but could be something in there.
depending on the VPN service you could probably bypass it that way but add some amount of latency?
You either need to get the phone to do routing, which should be technically possible but likely isn’t supported, and could possibly be specifically prevented as i can think of ways people would try to abuse that.
Or, you need some way of adding routing capability in, either by having it done through a VPN to whatever end point, or finding some way to share the basic data connectivity from the phone to a device that does the routing, though you might still run into stuff on the phone side.
You could do the latter with ipcop and having a wireless interface on RED connected to the phone hotspot, and even host ipcop on the same machine in whatever type of VM and use a virtual ethernet to connect the host OS through it. then ipcop would be doing routing for you. ipcop doesn’t support upnp but you can forward whichever ports you need to once you’re handy with iptables, so ymmv, if it works at all. Not sure if there’s a upnp supporting newer equivalent to ipcop, the world leaned heavily into ‘hardware’ routers running linux. I suggest this over using your existing router as i don’t know how you’d connect your existing router to the phone hotspot as that’d be running it’s wireless in two different modes and i’ve no idea if that works.
I don’t have a solution to offer, but i found it an interesting question…