You seem to imply the router you chose (Archer C7) only makes use of the ath9k driver, which is not the case for the 5GHz bands. Atheros 802.11ac chipsets use a driver (misleadingly) named ath10k. Ath10k is not a driver that has evolved from ath9k as the name suggests, it's a very different driver that needs a binary blob to work properly.

As a result, some of ath9k's makewififast patches cannot be implemented in ath10k. Some TP-Link routers use Atheros 802.11n chipsets and therefore ath9k exclusively. For those wanting to have a completely FOSS router with LEDE, I would recommend the model TL-WDR3600.