How about using the windows utility instead of TP-LINK client utility？

On the web interface of the router, there is one option to deal with that. You can go to System Tools →Manage Control, under Service Configuration, whether Remote Management has been Enabled or not....

Try to manually install the driver: http://www.tp-link.com/en/article/?faqid=451

As far as I am concerned, TL-R600VPN only supports site to site VPN, it cannot be used as VPN client.

what's more, use inSSIDer to see which channel is free to use and then choose that one as yours.

Have a test of the speed when the WMM is on and see whether it will make any difference or not. As to the problem in connecting the ipad, can you tell whatis the version of IOS?