IP address on network cameras change preventing remote access
You need to do two things:


First, make sure the cameras are not on DHCP (automatic addressing). This will need to be configured at the camera itself. You would need to assign them manual IP addresses as @gosmond already mentioned.
Next, make sure your Internet IP address is static. Very few ISPs provide static IPs for home connections (they do for business users). You can work around this by using a service like dyndns which will give you a domain name that will point to your router even if your ISP changes your Internet IP.