Opal GL-SFT1200 Captive Portal no Internet

Hello, unfortunately I always have problems bringing the router into a network with Captive Portal. I followed exactly the instructions from the Gli.Net document. I get a connection to the network and also an IP address but unfortunately no Internet connection. What else can I do here?

The SW-Version is 4.3.11

Individual smart phones can connect to the Captive Portal network without problems and also get an Internet connection.