Port forwarding.
First, make all cameras ports different from each other, for example:
HTTP: 8080, 8081, 8082, 8083
RTSP: 1554, 1555, 1556, 1557
e.t.c.
(each port you will find in settings)
Then, you can forward the ports through the router.
The feature is you need to have the same ports on the camera and on the router external IP. I did it.