Can you describe your WiFi setup? Specifically:
Brand of WiFi router or mesh system.
The 2.4 GHz channel being used.
The width of the 2.4 GHz channel being used, i.e. 20 MHz width or 40 MHz width?
Zigbee and WiFi share the 2.4 GHz spectrum. WiFi, being much stronger, can inundate zigbee signals. Some WiFi routers "auto-optimize" the 2.4 GHz channel being used, which coincidentally can swamp out zigbee signals. I want to eliminate this as a possibility for the issues you have described.
This article will describe the issue in some detail:
This figure summarizes the issue of overlap. WiFi channels 1, 6, and 11 are shown here with a 20 MHz channel width.
