Hey @joebalazs,
Unfortunately, for what you are specifically looking for, it likely won't work the way you would like.
Technically, you will have access to 3 SSIDs: for the IoT Network, Main Network, and Guest Network. (Note: Of the Two Listed, only the X55 supports an IoT Network)
The SSIDs that you have indicate that they were only using one of the wireless bands, whether it be the 2.4GHz or the 5GHz networks.The SSIDs for Decos do not allow you to select a specific band for the entire SSID and the different networks are really mean to be used for their designated purpose. It is likely that your devices will not transition to the news SSIDs smoothly, as the 5G devices will be connecting to a network with both 2.4 and 5Ghz combined. You would then need to configure the signal preference (2.4 or 5) from within the Deco App.
For your needs, you will likely want to look towards our Archer series of routers - as these will provide the settings that you are looking for. If you are looking for a mesh solution, many of our Archer Routers also support EasyMesh networking, which will allow you to create a cohesive network without a designated "mesh system"