Here in El Salvador, most houses that use TIGO as their ISP have the same ARRIS TG1672G router. This router has a seemingly random password, but it actually follows a pattern thats really easy to guess

All we need is the BSSID of the WAP and the SSID. In my experience, its been the case that every home has the same model modem. I'll assume its the same for you.

Without being connected to a wireless network, run the following in a Windows Command Prompt as administrator.

C:\windows\system32> netsh wlan show all

  SSID 20 : ARRIS-81A2
    Network type            : Infrastructure
    Authentication          : WPA2-Personal
    Encryption              : CCMP 
    BSSID 1                 : 98:6b:3d:61:81:a0
         Signal             : 24%  
         Radio type         : 802.11n
         Band               : 2.4 GHz
         Channel            : 1 
         Bss Load:
             Connected Stations:         0
             Channel Utilization:        35 (13 %)
             Medium Available Capacity:  31250 (1000000 us/s)
         Basic rates (Mbps) : 1 2 5.5 11
         Other rates (Mbps) : 6 9 12 18 24 36 48 54

That command will list out all local wireless networks. The first byte of the NIC Specific is "61" and the last part of the password is final 4 of the SSID.

Putting it all together, the password will be TG1672G6181A2

If this oversite doesn't get patched out in the feature, the only thing you may have to change is the model number of the password.