AP Isolation

So "connected to the gateway"? You are referring to being on a client on the 192.168.12.x?24 network or what? My guess is the client obtains its routing information from the 192.168.12.1 gateway router. The gateway router will not have knowledge of the 192.168.2.x/24 network and there is no administrative right to add a static route to that segment on the Nokia router. Start with the client and if it is a Windows client issue the route print command. If you do not see the route there it has no way to get there. You can do a route add to the client(s) to inform it/them how to get to the 192.168.2.x/24 segment and what router to use as the interface to get there. You would need to add the -p to make it persistent or it would not remain after a reboot of the client. I am assuming this is a Windows client. You can do the same with Linux or Apple iOS. I forget the syntax nuances. I just look it up when I need it or check help/man files etc… 

Be sure to add the -p option to the command to make the route persistent

route -p ADD 192.168.2.0 MASK 255.255.255.0 192.168.12.200 <rtr interface?

After you complete the route add issue the route print command again. Keep in mind if you have the clients on the 192.168.2.x/24 segment with a router directing the traffic out to the internet via the 192.168.12.1 next hop router sure they can start conversations outbound. Nothing from the outside will be able to initiate a conversation with any of those clients as the GW router interface has no knowledge of that segment. 

All this makes the assumption:

192.168.2.x/24 NET--Rtr INT- Routing Device -Rtr INT--192.168.12.x/24--GW<192.168.12.1>

                                 (192.168.2.1)                    (192.168.12.xxx)