>having a private ip = definately not publicly reachable.
What component of your router prevents a packet with destination IP 192.168.1.2 arriving on the WAN interface from crossing over to the LAN interface and reaching a LAN machine with that IP? Hint: It's the same one that prevents IPv6 packets from making that same crossing.
nothing stops 192.168.1 crossing a wan interface, in fact I and most of the internet rely on being able to do exactly that, the router just needs an appropriate route in its routing table.
What component of your router prevents a packet with destination IP 192.168.1.2 arriving on the WAN interface from crossing over to the LAN interface and reaching a LAN machine with that IP? Hint: It's the same one that prevents IPv6 packets from making that same crossing.