Finding the Wi-Fi channels
Your channel is in the router settings (2.4 GHz Wi-Fi). A Wi-Fi analyser app on Android or a command on a computer shows the neighbours: on Windows netsh wlan show networks mode=bssid, on macOS hold Option and click the Wi-Fi icon. Channels 1, 6 and 11 are the most common. If the router picks the channel itself (Auto), fix it, or one day it will land on top of Zigbee.
Why 15, 20 and 25
A Wi-Fi channel is 20 MHz wide, Zigbee is 2 MHz, and Zigbee channels are 5 MHz apart. Channels 15, 20 and 25 fall in the gaps between Wi-Fi 1, 6 and 11. Channel 26 is furthest from Wi-Fi, but some devices run it at reduced power or not at all, so we do not suggest it. Distance helps too: keep the stick away from the router and USB 3.0. Coordinator picker.