@Beebader,

A few things to try first

If you think it may be caused by the combine 2.4 and 5GHZ networks:

Set the devices up to use your IoT network if your Deco model supports it.

Otherwise, disable the 5GHz network on your main network, setup the cameras, and then reenable the 5Ghz network for your Wi-Fi.

If you can see the cameras in your list of networked devices, disable Mesh Technology, Fast Roaming, and Beamforming.

You might need to reconfigure the cameras to accept your new credentials if you did not reconfigure them when replacing your router with a Deco. 

Have you tried resetting the cameras to defaults or reconfiguring their network connections from scratch?