Problem with vehicle WiFi network

We got a PX4 Vision DevKit and now we have a problem with the vehilce WiFi network. When I try to connect the vehicle to QGroundControl, we use SSID: pixhawk4 and Password: pixhawk4 but we do not get a working connection to the WiFi, we tryed to connect to the pixhawk4 WiFi with different laptops and smartphones, but it always says “the connection is limited” and we get a yellow warning triangle on the WiFi Symbol on our device and QGroundControl is not able to connect to the vehicle. This problem was not there from the beginning, three weeks ago, we had no problem to connect to the pixhawk4 WiFi and we were able to use QGroundControl. As we did not change anything on the pixhawk4, we have no idea why why we are no longer able to get a WiFi connection to the pixhawk4.

Does anyone know how to solve this? We are thankful for any kind of help.