How to access different devices in the local network from external and 1 ip address?
To forward the ports. Access from the Internet is only possible if you have white IP address. If just from the external network, then not necessarily.
How to make routing between them, so I could work with every device?
Routing has nothing to do with you NAT.
Just probressive for each camera its own port. Then connect to the camera specifying the port, and all.
For example you have three cameras on the local addresses 192.168.0.1, 192.168.0.2, 192.168.0.3. Access to the camera is on port 4000.
So you just probressive 50001 on address 192.168.0.1 and port 4000 port 5002 to the address 192.168.0.2 and port 4000, and so on.
When you need to get on the first camera подключаtтесь - vneshniy:50001