NAT is not a barrier for traffic, so question 'how traffic get through a NAT' does not make sense. Router by default forwards packets. NAT just means that in so

NAT is not a barrier for traffic, so question 'how traffic get through a NAT' does not make sense. Router by default forwards packets. NAT just means that in some cases it also translates src/dst addresses of packets. If you have a router with NAT and without (stateful) firewall or RPF, and such router receives a packet on wan port with dst address from your private LAN range, then such packet is just passed to your LAN (as NAT does not apply here). Obviously, such packet is unroutable on general Internet, but could be send by our ISP or by other customers of your ISP if they are on the same network (e.g. wifi AP) as your wan port. There are also more sophisticated ways, like abusing automatic decapsulation of packets (if enabled). Fortunately, in most cases a router with NAT also has enabled stateful firewall, so this is moot.