So when I found the DAP-1650 range extender was released (I believe the only simultaneous dual-band, access-point configurable, range extender available), I quickly ordered one. It worked so well, I purchased a second one. I now have the DIR-825 router in the basement configured with channel 11 (2.4Ghz) and channel 40 (5Ghz).

https://www.amazon.com/D-Link-Wireless-Gigabit-Extender-DAP-1650/dp/B00JFOP688