To request for a feature, you'd better report it here. :) http://ticket.tp-link.com/index.php?/Tickets/Submit

Daniel, you have to figure out if the connection drops when modem is connected to PC directly. The router cannot do what you want...It is not as smart as your expectation...

Your old TP-LINK is not a dual band one, right? Detachable antennas on W8980 are 5GHz ones. How about yours?

You can use C9 as an access point and configure its IP to 192.168.0.2 to ensure all your clients can talk to each other. http://www.tp-link.com/en/article/?faqid=417

Screenshot pls. Are you connecting your PC to W8980 by a cable? If the PC has Wi-Fi function, ensure Wi-Fi is disabled or disconnected. By the way, check default gateway on your PC. Is it 192.168.1.1?

Configure one WDR4900 as the main router and enable WDS on another WDR4900. http://www.tp-link.com/en/article/?faqid=440 2 WA901ND can be configured in repeater mode.