I'm not clear what network setup you have. Are you saying that both the phone and the tablet are connected to the wifi extender ? Is this extender in turn connected to the main wifi router and is that router connected to the PC ? Are the tablet and phone both using the latest version of the BridgePal app, and the PC using the latest version of BridgePalServices ?
The PC sends its address over the network as a broadcast message and a multicast message (in an older release of BridgePalServices it used to be just a broadcast message, but some devices cannot receive broadcast messages so multicast was added as well).
If the Fire7 can only receive multicast, and if the wifi extender blocks multicast messages but not broadcast messages, then that would be a possible explanation. However, that's just a guess I'm afraid.
If you cannot get it working then you will have to manually configure the address of the PC into the BridgePal app. You could also setup the PC to have a static ip address (beyond the scope of this reply, but you should be able to find a guide via google search).
