From your question, unless you phrased it incorrectly, it is pretty easy to summize the problem here...
It works only for 1 to 2 hrs, then it cannot be accessed from LAN or WAN.
If you cannot access it from the LAN, this is not a router or port forwarding issue, but a camera issue. You verified this yourself...
If I use same port with my other device, it works fine.
The answer to your problem, not your direct question though, is your camera is defective or there is some other issue with it or it's connectivity. You did all the proper troublshooting, just came to the wrong conclusion. My guess is replacing the camera will correct the problem.