PSA: How to set up a hotspot on Windows 10 sharing internet that you get from wifi, when Windows 10 says "We can't set up mobile hotspot."

TL;DR

How to set up a hotspot on Windows 10 sharing internet that you get from wifi, when Windows 10 says "We can't set up mobile hotspot."

This assumes you have two wifi interfaces.

1. Install mHotspot (a third party app)
2. Go to the start menu, right click mHotspot, and "Launch as Administrator"
3. In the "Internet Source" dropdown, select "None"
4. Start Hotspot. Make sure your devices can connect to it, even if they don't get internet from it. If they can't connect, fix that first.
5. Go to the Settings app, under Networking & Internet Settings, click "Change adapter options". It will open the Control Panel that lists all your network adapter, including your Wi-Fi devices.
6. When you start a hotspot with mHotspot, it creates a `Microsoft Hosted Network Virtual Adapter`. Identify which it is. To do this, turn off the hotspot in mHotspot, check what devices there are, and then turn it on again. It should delete (when turning off) and recreate (when turning on the hotspot) an adapter. That's the one that you want to share internet *to*. The icon that disappears and shows up will be called something like `Local Area Connection* 10` in black text, and it will have `Microsoft Hosted Network Virtual...` written in gray text next to that. That's the one you want.
7. Now go to the properties for the network adapter you want to share internet *from* - in my case it's `Wi-Fi 2` - and under the `Sharing` tab, share internet with the `Microsoft Hosted Network Virtual Adapter` that mHotspot sets up. So from the drop down, you want to select the `Local Area Connection` that we identified in the previous step. Enable sharing and click OK to make it work.
8. Your other devices should be able to get internet from the hotspot now.

Long version:

Here's how I arrived at it:

after about an hour in chat and on the phone with Microsoft support, I have been able to find a workaround for this issue. It's not perfect, but I have been able to make do for now. Here's what I learned:

1. The support agent suggested that I use a third party app for setting up the hotspot. They supposedly do it as a matter of course if the built in Windows 10 feature doesn't work for some reason. I guess there's a bunch of issues with it.
2. The hotspot functionality only works if you are getting internet via a source other than Wi-Fi.

So while we were talking, I remembered that I used mHotspot in the past but it didn't work for me. The issue with it is that it doesn't show my wifi adapter as an internet source in the settings.

However, I started playing with it during our call. At some point, I set up mHotspot to use `None` as the internet source, and then I enabled the hotspot. My phone was able to connect to the hotspot now, but it had no internet. I noticed when I set up the hotspot, mHotspot creates a `Microsoft Hosted Network Virtual Adapter` and a `Local Area Connection` for it, called `Local Area Connection* 10`. So I went to my wifi connection that I get internet from, and set up sharing to `Local Area Connection* 10`. That fixed it.