In your situation, you could just pick up a basic, consumer. off-the-shelf router and set up your own home network. It's not necessary to have it connected to the any ISP, you're just creating your own, isolated home network where you can connect all your devices (phones, tablets, computers, etc.) so they can network with each other. Connecting your router to any kind of online access (i.e. broadband supplied from an ISP) is only necessary to access the Internet, it will work out fine as long as you're only trying to connect your devices with each other. These kinds of networks have been around way before the Internet was even a thing, and it's still something a lot of businesses continue to rely upon where thousands of computers are linked with each other, with online access being a very limited to only isolated situations. You can do the exact same thing in your home, set up your own home network where you connect all your things just to your router, and whether or not your router has any access to the Internet won't have any effect whatsoever on your devices connecting with each other within your home network.
But this is just a suggestion to free up that phone you're using as a hotspot so you'll always have your own WiFi network whether that phone is available or not. And it might be overkill, for a lot of situations you might be fine just using Miracast.
And as a side note, Chromecasts are an outlier as they do require at least some Internet access to even function. At one time, the original Chromecast was able to work just within an isolated, home network with no Internet but since that time firmware upgrades put a stop to that (they automatically receive OTA updates) so now any Chromecast (old or new versions) needs to ping Google servers to work. So if your TV doesn't have it's own integral WiFi capability, something like a Roku will work out better in this matter.