Does it happen after you upgrade the firmware? Did you disable anti-virus and firewall to try?

As I know, only some TP-LINK VPN routers can work as VPN server. Obviously, WDR3600 is not one of them.

Your MR3220 is V2? Did you upgrade the latest firmware? If you did but no help, I'm afraid that there is no any good solution to it as none of them are in compatibility list.

Wi-Fi name of all extenders are the same or different from each other? http://www.tp-link.com/en/download/TL-WPA4220_V1.html#Utility Did you try the default Wi-Fi password on the back of the...

Powerline adapters are also TP-LINK? Did you try to connect directly to LAN ports of C7 and check the speed? When connecting to 5GHz Wi-Fi, what's the link speed showing for wireless network...

Tony Seaford wrote Disconnect modem from WAN port of C7. Connect PC to LAN port of C7 and check IP address obtained after you reset the router. Did you try it? What's the IP address? Provide a...