Copying here hat I put in Super User a few days ago:
I had a peek at the not-too-informative manuals and the not-much-more-informative website. I did notice your setup is the same test setup you find in the installation guide. There's the same test, ping from laptop connecting to the Client to the laptop connecting to the AP. If it works, test is successful; there's no mention that pinging the other way should be successful or even tested.
There's four modes; AP, Client, Master and Slave. AP is clear, Master/Slave combo is for creating a wireless bridge (WDS). I guess the Client mode is intended for using this device as a dumb wireless NIC.
In that mode you'd connect the box to Ethernet NIC of another device which has no WiFi capability. The Client is the one which connects to the wireless network, just as a wless NIC does, and acts the same way - passes traffic between the wireless interface and the "bus", i.e. the Ethernet interface. Since the other device is not actually connecting to the network itself, it won't be seen and it shouldn't need an IP address at all.
Run a Wireshark trace on laptop A when you're pinging from laptop B. I bet the ICMP echo packets are originating from IP address .127.254. If I'm right, from the network point of view address .127.1 doesn't exist at all.
I think that if you want specifically laptop B itself to be pingable, you'd need to configure Master/Slave mode. However there's a catch: now .127.253 is in AP mode, which means any wless client can connect to it.
The specs specify two antennas, but I couldn't find out how many radios the device has. One radio can only perform one function, it's either serving the WDS bridge, or serving wireless clients. So if there's only one radio, the Master and Slave can only talk to each other, not to wless clients.
Please do let me know :-)