Aha, that's the problem. Go to the configuration of the router. In some routers it is also possible to do a PAT = port address translation. Then you can also change the port used with the public IP to an internal port on the LAN. For example publicIP:1234 change in the router to lanIP:88. So check the router. If that is not possible you must change port in the camera.
According to the manual it is posdible to change the port number.