Nullity
Very Senior Member
My N66U has always lacked some of the 5 GHz channels that are available in the UK, whereas my AC68U has those channels available and that's with both routers running the same firmware version and without any modifications by myself. Both are UK models. The main difference I've noticed is the bootloader version; the AC68U has a newer CFE than the N66U.
With my RT-N66U, while experimenting, I enabled 1-13 on 2.4Ghz and a dozens on 5Ghz. I do not run it that way, but it is possible.