WN3500RP issues with 5ghz speeds and question about wired connection

I recently upgraded to 100mb/s broadband and when I connect to my broadband via the wifi extender (WN3500RP) my speeds are terribly slow, my download speeds only reaching a few hundred kilobytes. I know that isn't right, but this only applies to the _5GEXT not the _2GEXT. I then plug in my ethernet cable to the WN3500RP and plug it into my computer and I achieve speeds I never have before, 7 megabyte download speed but it says that it is _2GEXT; how can I make my ethernet _5GEXT? And how can I fix my _5GEXT wireless connectionn from the extender? (hoping thats possible).

Thank you ;)