Solution 1:
Mobile devices switch back to data network mode after connecting to the gateway during the installation phase.
With some end devices, it can happen that the end device reconnects to the mobile data network after establishing a connection with the gateway. Reason: If mobile devices cannot access the Internet in the WLAN, they may automatically switch back to the mobile data network. Some manufacturers indicate this and ask whether the connection should be used despite the lack of Internet. Other devices like e.g. IPhone do not ask, but switch back automatically. This can be recognized via the menu bar of the smartphone by the WLAN symbol or data network mode. At Apple there is the possibility to activate or deactivate the point under "Mobile network" à "WLAN support" (automatically use mobile data in case of bad WLAN connections). We recommend to deactivate the point during the initial setup.
Solution 2:
Router with WLAN 2.4 and 5 GHz
To operate the gateway, it is mandatory that the gateway and the mobile device are in the 2.4 GHz network. We recommend to disable the 5 GHz network, because the mobile device cannot connect to the gateway if it is in the 5 GHz network.