Solution 1:
Fritzbox does not allow WLAN devices to communicate with each other
Some Fritzbox models have an option in the settings in the menu à WLAN à Security, "that WLAN devices are allowed to communicate with each other". This option must be activated.
Solution 2:
Affects all router models - Assign a fixed IP address
The gateway is automatically assigned an IP address via DHCP, this is not fixed and can change at will when the router is restarted. Remedy: In the router menu, permanently assign the IP address for the gateway.
Solution 3:
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 switching off the 5 GHz network, since the mobile terminal cannot connect to the gateway if it is in the 5 GHz network.