Download and install button not working

That greyed out button is telling you exactly what the problem is.. right below the button.. did you read that?

You need to connect to WiFi. (Not a hotspot)

Alternatively, you could use a computer that is connected to the internet.

Update your iPhone, iPad, or iPod touch - Apple Support

You'll need to find a WiFi network or a computer you can connect to in order to update. Cellular data and hotspots can't be used. If the iPhone was a 12 or a 13 on a 5G plan, then it could possibly update using cellular data.