AP Isolation

Routing is a two way street. The router interfaces on both networks must be aware of one another. Sure you can't influence the router gateway as it is set to communicate to the external public net via the NAT translation. You need to establish a network router interface on 192.168.12.x/24 for the communication to 192.168.2.x/24 and have that router have directly connected interfaces on both networks. For router on the 192.168.2.x/24 & 192.168.12.x/24 it need the next hop gateway on that to be 192.168.12.1 to reach the internet. The any network 0.0.0.0/24 to go to 192.168.12.1 i.e. any network it does not know can only be available via the 192.168.12.1 gateway address. I would guess you have a asynchronous routing issue right now.