How could LN mitigate NAT transversal problem? As we all know, IPv4 address space is already exhausted, many ISPs have deployed carrier grade NAT (CGN). So how could an LN node living behind NAT deal with NAT transversal problem? How could two NATed LN nodes communicate with each other?
first of all if your lightning node sits behind a NAT it can still connect to other lightning nodes and also open channels with them. once you have channels you don't need a public IP address but your node id is enough. your channel partner and nat will solve the rest. 

The problem is that your node behind a NAT does not have a public IP address. therefor you need to configure port forwarding in your NAT.

edit:
Also you can run your lightning node behind a toronion to have a static address.