Internet - Verizon Fios Gigabit, ONT directly connected to WAN port on TP-Link Archer C7 v2 in downstairs room. No Verizon router hooked in (but I have one for a month or two to test with if it's necessary before I go down to the 100 or 300 Mb packages and it isn't free anymore).
Wired LAN - Cat 6 run from main Archer C7 to Archer C9 in upstairs room.
WiFi 1 - Same SSID on 2.4GHz and 5GHz on Archer C7 downstairs. Channels set to 11 40MHz width and 165 Auto Width.
WiFi 2 - Same SSID as WiFi 1 on both 2.4 and 5 GHz, on Archer C9 V3 upstairs. Channels set to 2 40MHz width and 36 80Mhz width.
Both routers are set to high transmit. There is very little interference from other houses, just a couple occasional channels at less than 80 dBm, and then the WiFi direct Fire Stick on whichever it's connected to.
I have two main issues.
1) The upstairs C9 wireless is very slow, on either 2.4 and 5 GHz I'm lucky to get 20 Mbps, and the latency is atrocious, loaded latency of over a second sometimes. Wired to the C9 directly I have hundreds of Mbps up and down and latencies under 30ms loaded.
2) The upstairs C9 5 GHz signal is extremely weak. No matter if I'm 2 feet or 50 feet from it, I'm only getting about 75 dBm. It's overpowered often by even the 5GHz from all the way downstairs.
I did a factory reset of the C9, checked that the firmware was up to date. Neither helped.
When it was the main router (cable comes in there, before I switched to Fios) and I was using WDS to link to the C7 everything seemed fine with the C9.
Even once I swapped where the internet came in but was still using WDS until I ran the cable yesterday, I was getting better speeds from the C9 as the WDS router, it just wasn't super reliable, and obviously not as fast as it could have been.
I'm out of ideas, unfortunately.