What you mentioned is LAN to WLAN speed instead of Wi-Fi speed, isn't it? You tried any other router with a better LAN to WLAN speed? Maybe it is the limitation of clients themselves.

No difference in Wi-Fi range of 2.4GHz Wi-Fi and 5GHz Wi-Fi range is shorter than 2.4GHz. If you just need to extend Wi-Fi range, no need to buy a router. Buy a range extender or powerline extender...

It cannot be changed because it will be the same as the main router's channel if it is configured successfully like it displays on Status page... What's the version of inSSIDer you tried? What does...

Umm, you can try to add the correct network profile manually in your house and select connect automatically. You'd better have a test in your house first with your router.

Both D7 and D9 supports VPN passthrough and IPsec VPN only, which doesn't seem like what you need... How did you make it work in your existing router? Do you remember it?

As I checked its specifications, it supports VPN passthrough and IPsec VPN. http://www.tp-link.com/en/products/details/cat-15_Archer-D5.html#specifications What kinds of VPN connection do you need?...