On web interface of R600VPN and Advanced Security page, did you uncheck Block Ping Packet from WAN Port?

You are trying to open port 3389 for 10.x.x.7 on ER604W and access it remotely via remote desktop connection, right? Protocol is TCP? WAN IP is public?

Your extender is WA830RE? The firmware is up to date? Can you connect to LAN port of the extender and try to download the file? Did you test and compare the speed connecting to the root router...

How did you configure the range extender and confirm it has been configured successfully? Follow the guide below to do troubleshooting. http://www.tp-link.com/en/faq-691.html

TL-WN851ND is not compatible with Windows 8.

Internet Access is 3G/4G Only? The USB dongle is always plugged in even MR3220 is turned off? After reset the router, select correct Location and ISP. After Save it, wait for a while and check if...