Diagnosing problems with specific wifi clients

I have 2 or 3 out of probably 15-20 WIFI client devices that have problems with my LEDE/OpenWRT routers. The devices with issues are a Roku TV and 2 different Amazon Fire TV sticks (2014 and 2016 generations).

iOS devices, windows laptops, etc. are fine. A Google Chromecast in the same location is also fine. The devices connect but then disconnect and/or have really poor throughput, pings wind up with multi-second RTTs with packet loss.

I've tried two different router/APs, a TP-Link TL-WDR4300 4300 hw 1.3 and a RouterStation Pro with an ATH5K card. I've tested LEDE Reboot 17.0.1, Chas Calmer 15.05.1, and Barrier Breaker 14.07.

If I swap in an Asus TM-AC1900 (a rebranded RT-AC68U) running the stock firmware ( everything is fine.

I unfortunately don't have good notes, but I seem to recall things got worse somewhere around the Barrier Breaker -> Chaos Calmer releases.

I'm sorry this is fairly vague, but I don't know how to go about diagnosing this so that I could begin to formulate a meaningful bug report or correlate this to any existing bug reports. I've turned up hostapd logging. Sometimes i see 'disconnected due to excessive missing ACKs' and 'EAPOL-Key timeout', but I don't see enough of those to explain all of the problems. I'm guessing these would also be seen if there was just a very marginal RF connection. The signal levels don't look that bad from LuCI, though the data rates look very low.

I can share logs, provide more hardware details, etc. but I need some guidance on how to proceed in diagnosing this.

Thanks for your help.

Since you have them easily available on different devices, I'd start by sniffing your beacon frames from the TM-AC1900 and comparing them to the LEDE AP beacon frames. That'll tell you if something is parameterized differently and give you some leads to look into... e.g. maybe the devices choke with/without .11h country code or something.