You can open port 21 on your main frouter for WDR3600 to make it work. Or you can set WDR3600 as a DMZ host on your main router, but it will be unsafe

Maybe it's the problem of your USB drive, what's the speed when you connect directly to your computer?

Have you guys enabled "broadcast SSID" on the web utility? And the antennas of WDR4300 should be dual-band ones, if you want replace them , you should replace with dual-band ones. Otherwise it may...

Apparently your WDR4300 is behind another router, maybe your main router/gateway, 192.168.0.3 is a private IP address. If you want to make it work, use WDR4300 as the Internet gateway , or open ports...