To configure your nanoleaf devices on what looks like a 5GHz-only BT WholeHome mesh network, you need to work around the network using another device. This might be your router by disabling 5GHz or a Mobile Hotspot. For example, in my case I:
1) Turned off the BT WholeHome network
2) Created a new mobile hotspot on one phone running at 2.4GHz only and with the same SSID and passcode as my WholeHome network. This is essentially a mobile hotspot mimicking the WholeHome network, but at 2.4GHz.
3) Joined this network from another phone running the nanoleaf app
4) Configured the nanoleaf light panels using the nanoleaf app, so they too joined the mobile hotspot network.
5) Turned off the mobile hotspot.
6) Turned the WholeHome network back on. The mesh network does in fact support 2.4GHz as well as 5GHz and the nanoleaf panels now connect to the mesh network.
I had to do this because of limitations in my (non-BT) router. You could do the same from the router if you don't have such limitations.