The WhatsApp on the smartphone of my wife shows by and then on the left above a WLAN symbol with a question mark and the request to open a session in the net, although it is connected to the WLAN. Other apps do not have any problems using the WIFI, but WhatsApp asks for the mobile data connection. I noticed the problem when her mobile data was used up after a reset of the smartphone (but not a factory reset). Do you encounter a similar problem? That WhatsApp is eager to use your GBs instead of using an existing strong WiFi connection?