The best way to separate the two networks is with the following procedure:
1- Go to your HelixFi mobile app
2- Go to the Connection tab
3- Go to View Connection
4- Click on the white pencil logo at the top right corner of the screen
5- Check the box "Use different names and passwords for 2.4 and 5 GHz"
6- Create a 2.4 GHz network name and a 5 GHz network name and two passwords.
7- Then, don't forget to click on Apply.
8- Finally, try to connect your devices on 2.4 GHz network you created and for your other devices, connect them on the 5 GHz network.
Hope this will help! Have a good day!