With regard to AC68U and Smart Connect, I just was just on the phone with Sprint because I was having issues with WiFi calling with my Galaxy S6. They were suggesting I use their router that was designed to work better with WiFi calling, in part because it has a Smart Connect feature. The router that they use is an AC66U. Here's a link to details on their router - it does have some customized Sprint firmware - maybe to provide the Smart Connect - but point is they have it working on the AC66U; seems like the AC68U would be capable as well.
They use a newer SDK version than Asus for the RT-AC66U. It's not just a simple flag to enable at build time, you need to change to that newer SDK.