There’s usually an option in the setup for your router that lets you choose the channels it will use. I don’t know where this is on your ATT router.

The problem with choosing the best channel, especially at 2.4 GHz, is there are many types of devices using those frequencies—not just computers. So, doing a network scan doesn’t give you a real picture of the congestion. For example, I have a wireless outdoor temperature/humidity monitor that’s connected to a receiver in the house. It’s using some frequency in the 2.4 GHz band, but I have no idea which one. Baby monitors, intercoms, intrusion alarms, etc. are other devices that might be using the 2.4 GHz band, but they won’t show up when you check for WiFi usage.

This is exacerbated by the fact that adjacent 2.4 GHz channels can partially overlap each other. This means if you’re on channel 1 and someone else nearby is on channel 2, your signals can interfere with each other.

Without special equipment you can’t really tell which channels are the least congested. Trial and error is maybe the best approach. On 2,4 GHz, I’d start trying with channels 1 and 11 because they’d only have possible overlaps on one side.