Looks like an autoreply from FurryNutz, and I tried to pre-answer all these, but just to be complete I'll go through point by point:
- DIR-860L rev A1
- Firmware 1.08 (originally 1.05). Interestingly both had the same behavior.
- North America (California)
- TWC, but that's irrelevant here as my WAN works fine
- Again should be irrelevant, but I have a new ARRIS / Motorola SB6121 SURFboard. Works great.
- default SSIDs change? check! Bonus: SSIDs are nice clean ASCII (no special chars except the dash in the dash-five-g-h-z network)
- currently 2.4GHz = Mixed N+G+B and 5GHz = Mixed AC+N+A, but I've tried simpler versions like 2.4=B+G and 2.4=B, and that doesn't change things.
- Width: Currently 20MHz-only, but I've tried auto as well.
- Channel: I started with auto, but now I'm static at Channel=1
- Security: WPA-Personal, I've tried various security modes with that and currently on Auto
- Currently just a MacBook Air (Mid-2013), a Lenovo X230i, an iPhone 4S and a 5S, and a Samsung Galaxy S4, but I have lots of other machines I can hookup if I want.
- Any 2.4/5 devices nearby? Nope. I own a Uniden 5.8DECT phone that I've tested and it does not interfere as far as I can tell (remember my 5GHz network is the one that works!)
- ~10 networks nearby, no particularly strong or crazy signals, though. Pretty calm for a mid-suburban RF environment
- I don't see a Short-GI option for my device. WLAN partition is already off, I don't see "extra wireless protection" and HT20/40 coexist is already disabled (which is default from what I can tell).
- Router is placed in the middle of an open room on top of a table in floor 2 of a 3 floor townhouse. I've direct just about every spherical angle and distance, I assure you that's not my problem because it isn't changing things at all.
- 3rd party stuff, virus protection, etc? All off and all irrelevant. This is clearly a 2.4GHz radio issue, not an internet or computer issue.