In the last week I bought one of these routers to replace my Smart Hub but found as the 'wizard set it up, my download speeds dropped from 51 to 48 Mbps. Much searching of this and other forums ensued as I was baffled because the router was showing sync speeds that should have resulted in identical performance, especially as the cabinet is 200 yards away.
Eventually I chanced upon a post on this forum about the VDSL Profile being set wrong for BT. Sage advice seemed to indicate that although the router defaulted to 31a it shouldn't matter (BT settings should be 17a).
Having tried a number of other changes, I finally decided that it shouldn't do any harm to set it correctly to 17a. Immediately I got the download speed back up to 51Mbps+
Here are my settings and stats should they be of use to other users of this router:-
I hope this may help other Asus DSL-AC68U users.
Andrew Johnson
Before anyone pops up to say that the above log shows an Interleaved download, I've just spotted it myself. I can only put it down to the DLM seeing one disconnect too many when I disconnected all tech from the phoneline while I was out as a precaution as thunderstorms were forecast. 
Previous near lightning strikes have induced voltages in the overhead phone line that have taken out a dial-up modem, sound card and a Sky Box one one occasion and a router on another. It's funny that the bog standard telephone stuff still works even now.
