See this issue on 5GHz only SSIDs too, so this does not appear to be related to 2.4GHz channels 1,6,11 channels only. That being said, 2.4GHz channels outside of 1,6,11 should never be used. If the fix for you all on older 2.4GHz (802.11b/g/n) technology is to move to a channel that should never be used in the US (i.e. channel 9), there are some serious engineering issues at Apple. I think they were trying to get you to not use channel 6, but should have never advised use of channel 9... There are some other long standing Apple best practices, outside of not using channel 6, such as:
- create a 5GHz only SSID, SSID that has 2.4GHz disabled. This prevents jumping back between bands and 2.4GHz typically works very poorly as compared to 5GHz in dense WiFi deployments (businesses, apartment complexes, high rise city buildings). 2.4GHz can travel further and better through numerous walls, so use may be needed in some larger residential deployments with too many walls or other RF obstacles in the path.
- Make sure TKIP and AES and not both enabled. TKIP is a deprecated security protocol, and enabling both on an SSID has been known to cause issues with Apple products. TKIP should never be used, with or without AES
- If you are able to do step one, creating a 5GHz only SSID, move everything possible in your home network to this SSID. This will enable better support to devices that only do 2.4GHz, i.e. gaming consoles and other devices still build on the older 2.4GHz technology.
- Try disabling "band steering" on your 5GHz, if your WiFi device is high-end enough to have this feature. This is a process that tries to force dual-band (5GHz and 2.4GHz capable) hardware to the 5GHz channel. The issue is that it sometimes has adverse impacts on 2.4GHz only devices.
- Disable channels 5GHz 149 and 153 if your wireless supports these.