I’m in a similar situation, in that I have an iPad and iPhone, but I do my Zwift running outside my home, so I do not have access to my home network. As you discovered, for security reasons, the iPhone personal hotspot will not allow the devices to see each other on the network, so the Companion App can’t work with Zwift on the iPad.
There is a solution. But it does require another device.
I recently picked up a Netgear Nighthawk LTE Router. It’s a small router that’s only a bit bigger than a phone. It contains a SIM and is a cellular device that you can add to your phone plan. It’s also fully configurable like a home router. But even without making changes to the settings, devices connected through the router are not isolated and can communicate with each other. iPad and iPhone both connect to the router via wifi and the router uses LTE data to connect to Zwift’s servers. So now I can use iPad and iPhone for Zwift running at the gym or hotel. It’s worked flawlessly and is a lot more immersive than the tiny phone screen.
I’m including a photo of the device I’m using, though there may be other alternatives out there as well. If you’re interested, at least you know that this one works well.