WDrR3600 is not a switch but a router. Does it make a difference? What kinds of activity did you refer to? How did you "observe" it?

Archer C7 jumbo frames support? No! ONLY TL-WR1043ND V1 supports it.

Follow this guide. http://www.tp-link.com/en/article/?faqid=341

I left the ONT in route mode, but disabled DHCP and WiFi. The only client connected to the ONT is the TP-Link router, and its DHCP is on, wifi on, etc. It is working now, I have a separate local...

Cannot change the name on router page. MAC address can be used to recognize the device and block unknown devices,

When connection drops, how about Powerline lights on those adapters? Upgrade the latest firmware.