NAT is not an 'implicit firewall'. Firewalls do packet filtering, NAT mangles routing. These are different things.
If you're trying to solve the problems of "How do we stop anyone on the internet sending packets to grandma's open ports?", then the answer is a packet filter. Thankfully, must sane consumer routers come with these.
If you're trying to solve the problems of "How do we stop anyone on the internet sending packets to grandma's open ports?", then the answer is a packet filter. Thankfully, must sane consumer routers come with these.