@carolyn56,

It is relatively common to have a Gateway (Modem / Router Combo Device) given by your ISP and there are two ways to use a mesh system or another router with it.

Option One - Use your Decos in AP Mode

This will allow your Netgear Gateway to be used in its current state, with its routing functionality still managing the connections. However, you will lose many of the advanced features offered by Deco, and the Decos will operate more as a means of extending your wireless coverage.

Option Two - Use Decos in Router Mode, and Place Gateway in 'IP Passthrough' or 'Bridge Mode'

To use another router with your gateway, you will need to enable one of the two options mentioned above, as these options will disable the 'routing' portion of your Gateway - allowing your router or Deco to manage the connections and traffic.

See this FAQ for More info on the difference between the two modes: What’s the difference between Deco’s Access Point mode and Router mode?