I think you are mistaken in thinking that the 5 GHz network is working.
Try this on your phone:
- turn wifi off
- switch to 5 GHz only
- turn wifi on
When you switching wifi off, all networks erases from the list. And then, when you switch wifi on, which 5 GHz only, there should be no networks in the list. If you are connected to 2.4 GHz network and switch to 5 GHz only without turning wifi off, then your connection still be in place. Also, check SSID (name) of the network you are connecting to. For example my networks are: "my_net" for 2.4 GHz and "my_net_5G" for 5 GHz.