@ScottyP73,

You are likely correct, Many IoT devices use cheaper and older network cards (Since they do not need the advantages of new technologies). Newer Routers will have features such as Beamforming and Fast Roaming, which will help provide you the best experience overall, but can cause compatability problems for these older devices.

For IoT Devices you have a few options to get started.

If you are able to connect the device to your Deco Network initially, you can try to disable Beamforming and Fast Roaming from within the Deco App for the device. This will basically "dumb down" the connection to the device, which often helps with stability.

I would recommend configuring your IoT Network for your smart devices. This will be a separate SSID with separate network settings and will focus on increasing compatibility with devices using older network cards.

You may also consider changing the network security type, as some devices do not support the latest versions - however, I would recommend doing this for the settings on your IoT network so that your other devices can still use the newer encryption methods.

  @ScottyP73,

In that case, definitely try to use the IoT network with a lesser encryption method - perhaps even limiting it to the 2.4GHz band only. This will give the dumbest of smart devices the best chance of connecting and will at least allow you to slowly upgrade your house. 

I would recommend if you are upgrading your smart devices to begin checking into Matter-Compatible devices if you have not already. These are much more flexible, follow a standardized network/command structure for local commands, and can be directly added to platforms beyond the device's branded platform.