I've seen a few people asking about this where a device can connect via either 2.4 OR 5Ghz and they wish to control this.
If you want to restrict devices to only connecting to 5Ghz there is a way you can do it. If you look at the devices connected and more importantly, how they're connected you will notice a device uses different MAC addresses on 2.4Ghz to the one they use for 5Ghz. Once you can identify each one in your device list, you can then block the MAC address that you want to exclude.
My phone is really slow for some reason over 2.4Ghz but fine over 5Ghz so I identified it in the device list and saw what frequency is was connecting to and named it with "24" at the end, then when it did manage to connect via 5Ghz the device appeared as separate device (different MAC address). I could then block the "24" one so it would only connect via 5Ghz.
You do need to have it connect via both frequencies to be able to find it in your list. I found connecting at the edge of its range would usually result in 2.4Ghz (Stronger but Slower Signal) and standing right next to the disc should get you 5Ghz.
