Trying to Disable Router on AC 1900 Cable Modem

I am attempting to disable the router, so I can add a deco tp-link AXE5300 mesh system for greater coverage. When I attempt to log in to the router through routerlogin.net or.com, I'm told I'm not connected to my router, so I can't log in. Clearly, I'm connected to my router, as I'm able to get to the Netgear site along with all other internet sites. 

So, is there a workaround or is it ok to connect the mesh system without disabling my Netgear router? Thanks, in advance, for any advice/solutions.