I bought a TP-Link Archer C9 to use as a an access point (ethernet backhaul) on the third floor of my townhome. It's in conjunction with an Apple Airport Extreme (latest gen) router using FiOS gigabit service. When I first hooked it up, I was pulling down 280+ on ac devices. Uploads approached 500. Then, I briefly swapped: C9 as main router, AE as access point. Wasn't as happy with the AE coverage, so I switched back. Now the C9 is maxing out ~93 down with a similar up. I've played with all of the C9's settings (channels, after conducting channels scans to see what the neighbors were using; channel width; tx power), even creating a separate 5GHz network to ensure my ac devices were connecting at 5GHz. I'd really like to get back to the initial WiFi speeds I was experiencing, and have run out of theories on what I did wrong when I switched back.
I did not find the right solution from the internet.