Have a 3 unit AC1200 Mesh Wi-Fi system. So far I've opened the package and downloaded the app. It appears that this replaces the router? Can I not do that? I just want to place these 3 units at the far reaches of where the Wi-Fi dies in my house to extend the signal, and use the router I have. One unit as close as it must be (Does it have to stay connected to the modem?), Then one further away, then the 3rd the furthest away. Access Point mode, right? 

I can't proceed in the app without disconnecting the old router.