Zigbee channel 20 partly overlaps with WiFi channel 11 (in its sideband).
Source: ZigBee and Wi-Fi Coexistence
Channel 18 would probably be the one with the least interference from your WiFi. Alternatively, if you have the option to change the WiFi 11 channel to 6, then it opens multiple options for Zigbee in the higher set of channels. Homey will then typically choose Zigbee channel 25.
Changing the Zigbee channel on your Homey requires you to reset its Zigbee network. If I am correct you then also need to repair all your Zigbee devices (via the maintenance option in the device tile’s settings).
