networking - Home LAN with two subnets: only one subnet can see the other

I set up a home LAN with two routers, the second used to repeat the Wi-Fi signal of the first one in an area of the house where the signal of Router A doesn't reach. Both routers and all clients are connected through wireless connections; none of the routers has Ethernet ports.

Router A is a portable router that connects to the Internet through a 3G connection. The second router (Router B), also portable, is a wireless client of router A and also repeats its Wi-Fi signal.

Router A is a portable Huawei 3g router

Router B is a portable Hootoo Travel Mate router

The network is set up this way:

Router A

• Connects to the Internet via 3G connection, WAN IP and gateway assigned by the ISP


• 
  

  LAN address 192.168.8.1, netmask 255.255.255.0, DHCP disabled

  
  

  Clients:

  
  
  
  
  • .30 Android tablet (media server, Samba server)
  
  
  • .20 Android phone
  
  
  • .10 Windows 8.1 laptop
  
  
  • .2 Router B
  
  
  
  

Router B (Wi-Fi bridge/repeater)

• Connects to the Internet via Router A


• LAN address 192.168.9.1, gateway 192.168.8.1, netmask 255.255.255.0, DHCP disabled


• It's client of Router A with IP: 192.168.8.2 (wireless connection, not cable)


• 
  

  It's gateway for its clients with IP 192.168.9.1

  
  

  Clients:

  
  
  
  
  • Android phone
  
  
  • Windows 8.1 laptop
  
  
  
  

The Android tablet always stays on network 192.168.8.X; the phone and the Windows laptop roam between 192.168.8.X and 192.168.9.X. They have fixed IP addresses on 192.168.8.X because of a Wi-Fi backup system I have in place. The Android tablet hosts a UPnP media server (Kodi) and a Samba server.

From 192.168.9.X I can ping all machines both on 192.168.8.X and on 192.168.9.X.  I can also access the media server, even though it doesn't get automatically detected; I have to specify its IP address, I imagine because broadcast packets do not pass through router B.  I can access the Samba server through Windows shares but not by name, only by specifying the IP address of the share. That's not a problem.

From 192.168.8.X I cannot ping any machine on 192.168.9.X, but only Router B as 192.168.8.2.

I have no need of two subnets, but couldn't/don't know how to configure Router B to just extend the first network without creating a new one.

How can I configure my equipment so all my machines can talk to all other machines, regardless of their location in the house?