Originally Posted by ChrisR
It is a good idea to turn off all devices and test one by one.
Note that I faced similar issue using my bluetooth headset. My laptop was working ok for weeks, after it suddenly could not connect to 2.4 anymore. Using 5 was no problem.
Only when I switched off the bluetooth of my headset, the 2.4 connected immediately.
Therefore, turn off bluetooth for each device, make sure your microwave and home telephone devices are also switched off. None of these devices should be close to your router.
Edit: more tests: turn off all Household devices. Put your router at another place in the house. Last test: take your router/laptop to family/friends and check if problem still exists...
I found the culprit in my case, and it was kind of obscure. (Note that I am not the OP that had returned four routers for the same problem).
I have managed to connect to the 2.4GHz network now and then, but pinging the router gives a very flaky connection, and response times around 1000-2000ms. Browsing the web on that is not remotely possible.
Turning of equipment all around the house did absolutely nothing, and since the closest neighbor is 300m away the WiFi-space is far from crowded. Strongest other WiFi is at -92dB. Also, my previous 801.11n-routers has worked perfect, but I needed longer range to reach my new guesthouse.
I then moved the router from the closet it's been in, to the living room, and suddenly the 2.4GHz worked again! I put it back in the closet... and it still worked.
Now, I run the router in "AP Mode" since I have a Debian-based router I'm very satisfied with. Since the ports on the N66U falls back to working as a simple switch I took the oppurtunity to plug in the printer besides it, and this turned out to be what caused all the problems. When I plugged in the printer again, the ping latency rocketed to the thousands, and when I unplugged it, it went down to 3-4ms.
The printer is connected via an arcane printer server, so old that it only have 10Mb network (at the side of the RJ45, it even sports a BNC-connector for 10BASE2). For *some reason* plugging this in the ASUS completely crippled the 2.4GHz network, and for some other reason also did nothing bad to the 5GHz-part.
I don't know if it's because it's a 10Mb device, if it's because the print server puts out some nasty noise on the network cable, or if any device plugged in the ports will do the same. It would be interesting to know, but right now I'm too tired to test it. Also, I'm very happy because I wanted to keep this router.