Good morning Pamelas23,
When you say that you were able to disable the 5 GHz network, do you mean that you've created two networks like this?
Go to the HelixFi app on your phone
Go to the Connect tab
Go to See network
Click on your WiFi network ( just at the right of the WiFi logo )
Click on the white pencil logo in the top right corner
Check the box "Use different names and passwords for 2.4 and 5 GHz"
Create a 2.4 GHz network name and a 5 GHz network name. Make sure to create two different SSID's. Finally, create a WiFi password for those two networks.
Then don't forget to click on Apply.