The first thing to ask yourself: which system do you want to control the network? If your Orbi system is newer and you’re looking for seamless coverage across a large home, use Orbi as the main router. In that case, your Nighthawk AC1900 needs to be demoted to Access Point (AP) mode. https://nighthawkmesh.com/why-nighthawk-ac1900-gets-blocked-by-orbi-mesh/