@PINTUDAS,

This is a result of the mesh functionality of the Deco Networks. Splitting the SSIDs would effectively break the mesh network. There are options to select what band to use for the main network(2.4GHz or 5 GHz or 2.4+5GHz). 

To achieve the setup that you may be looking for, you would need to choose one of the option that is made up of only one frequency; then enable the guest network for the Decos, and then use that network for the 2.4Ghz connection. There should then also be options to allow devices on the guest network to see devices on the main network.

If you have no need to separate the bands and only need a designated 2.4 GHz band, I would recommend leaving the mesh configuration as is, and enabling the guest network. This would allow your mesh network to still function to its maximum capacity, while also providing a designated 2.4 Ghz band.