Most Wi-Fi routers default to what is known as a simultaneous dual band network that uses the same network name for both the 2.4 GHz and 5 GHz signals.
Wi-Fi devices like the iPhone and computers will simply choose the network that they deem "best".......which is often the 5 GHz signal since it provides a faster connection.
To do what you want......everything depends on whether your Wi-Fi router is capable of assigning different wireless network names to each of the 2.4 GHz and 5 GHz network signals.
If you could provide the name of the make and model number of your Wi-Fi router, we'll try to find out whether your router is capable of providing the options that you need.
Unfortunately, the Wi-Fi routers supplied by most providers do not allow many options at all, since the provider is trying to keep things as simple as possible......so they don't have to spend a lot of support time on the phone with their customers.