I did think of just setting up a Hotspot with another phone but it gets pretty annoying having to have 2 people at all time.
So I went digging in the Internet and found someone at a random forum (can't find it now for credit..), that said that If you set up the Hotspot in your own phone and configure it with another one you don't need the other one around anymore - being the Hotspot works as being connected to it.
Now I'm pretty sure there shouldn't be any reason I couldn't set it up on my own phone (the hotspot), except the Google home app just demands I connect to the WiFi network, which I can't do because you can't connect to yourself sadly.
I think if someone made some changes to the Google home app, or made an alternative set up app it could work. I have no idea how to code an app, so I came here 🙂
tl:dr I Want an app that does the same as the home app for setting chromecast up, but without checking if I'm connected to a WiFi network. (I think)