NAT1 configuration is simply a bidirectional relationship with your external IP and Internal IP.Thus,all incoming connections are allowed.
NAT2 configuration is also bidirectional relationship albeit some of the incoming ports are closed for added protection.Usually used by ISP which are generous.It requires Port forwarding but most router ship with inbuilt config allowing commonly used port for direct connections.
PS I am not very sure about NAT3 and NAT4.
Did your internet disconnect or smth after setting it to this? Because back then when I changed DNS in router, my internet got disconnected and I had to wait for about a week or two. And since it's lockdown rn, I'm worried I'll lose internet connection. Thanks!
Since NAT configuration is a router level configuration on user premise,there is no need to disconnect the router. Usually,the generic Chinese ONT allow us to change anything with real-time implementation.