If the phone only supports the 2.4 band then it can only connect to the router using that band. But other devices connect to the router using the 5 band as they support that. Except the Chromecast, which will use 2.4. They can all talk to each other via the router. You really don't need to worry too much about which band a device is using as long as it works.
Do you actually have a problem with something not working, or is it just the speed that concerns you?
Sovereignty really does mean sovereignty