
A Zigbee network uses one channel of the 2.4 GHz band, from 11 to 26. Choose a channel away from your Wi-Fi: see Zigbee and Wi-Fi, and measure the radio activity with Network > Interference level or a real-time interference scan.
When a zigpy coordinator (Texas Instruments, Silicon Labs, ConBee/RaspBee…) forms a new network with the channel setting left to default, it runs an energy scan and picks the quietest of the channels 11, 15, 20 and 25.
Some devices were reported to need a specific channel (findings from the ZiGate era):
There are two ways to change the channel.
The coordinator moves to the new channel and broadcasts the change to the whole network. The paired devices are kept.

The routers (mains-powered devices) receive the broadcast and follow. End devices (battery devices) sleep and miss it:
Wait a while, then check in Management > Device Management which devices don’t report any more.
The channel setting (WebUI Settings, Coordinator settings, advanced) is only applied when the coordinator forms a new network. Forming a new network erases it: all the devices must be paired again. Use this method only when you start a network from scratch.
Set channel to the channel you want and click Validate.

autoRestore setting off (Zigpy CIE specific configuration, advanced): otherwise the plugin restores the last coordinator backup, on the old channel, instead of forming a new network.autoRestore back on.With a ZiGate, the channel setting is applied with a coordinator restart (Admin > Coordinator, Soft reset) instead.
Go to Tools > Tools and click Coordinator info: the Channel field gives the current channel. See Use the plugin tools.