Android 11, Mobile hotspot, under advanced, has an option called "Wi-Fi sharing", where it says "Share your phone's Wi-Fi connection in addition to mobile data".
I've switched this on, set the phone to have WiFi on and mobile data off, and then switched mobile hotspot on. But it immediately comes up and says "The current Wi-Fi network can't be shared using Mobile Hotspot. Mobile data will be used instead."
If I click OK, I see I have WiFi still on, mobile data still off, and I'm surprised to find I can connect to this mobile hotspot, and reach the internet with it. (In fact this question is being sent over it...)
My question was going to be what kind of WiFi network will this work on, and that is still my main question, so I can guess when it might work in the wild (*). But I'm also quite curious why I got an error message telling me it won't work when it actually does. I am running a VPN on the phone - could that be what confused it?
*: Main use-case I can think of is when you pay for WiFi at a hotel but they limit to you to N devices; or free WiFi which involves a tedious signup process for each device.