AP Isolation

So, you have both wireless clients on the 192.168.12.x/24 network and they cannot ping one another? Are these MS Windows clients? If so I would guess the firewall is blocking the pings. You may have to provision an exception in the firewall interface OR just temporarily disable the FW on the two clients to confirm the suspicion. If you have IPv6 enabled on the clients then you could check and see if they are aware of one another in the multicast domain. The clients on the same segment don't need the gateway address to talk to one another but they should both have ARP entries to be aware of one another. I would guess FW blocking pings.