It appears you're trying to connect using QGroundControl from a phone or tablet with Android or IOS.
You only need a 5 GH WiFi capable device, and connect to your Mantis Q AP using its SSID and password.
Don't connect at all your controller.
Start QGroundControl and it will discover and connect. You need to configure video stream (rtsp://192.168.42.1:554/live ) and select virtual joysticks, they will appear on the main screen.
I use without any problem a Windows 10 PC with the corresponding QGC version:
View attachment 17228