@scottiedog1 The Deco app will guide you thru the steps to execute.
In the app, you must create a new network, and, during the process, you will connect the Deco to your existing router.

Be sure to define the wifi of the Deco with another name than the wifi of your router.

Once everything works well, you will perhaps decide to change the mode of the Deco, from Router to Access Point.