Configure el enrutamiento para que las conexiones LAN pasen por wifi y WAN por módem

Question

I have to configure my machine, so that packets for local machines (in the 192.168.. subnet) go through my wireless network, and WAN connections (any other IP address), go through modem.

Consider following example:

I open a web browser and type 192.168.1.4/my_intranet_website. I want to make sure that all traffic regarding this request goes through my wireless LAN interface. But when I type www.superuser.com (which is outside of my local network) I want this traffic to go through my USB modem.

I tried to figure out, which /dev entries stand for which network interfaces, but I cannot do even that. I remember, then when I used Debian, it was /dev/wan0 or /dev/eth0 for wireless/wired and /dev/wcdma0 for the USB modem. Here? No idea. The modem is an USB modem, so I think it may be one of /dev/ttyUSB0 and /dev/ttyUSB1.

Really sorry for such a laconic description of the problem, but that is how noob I am :/ I will provide any further necessary information.

Any help will be highly appreciated, thanks in advance.

EDIT: traceroute output:

1. When both (modem and wlan) connections are up:

   sopel@mediacenter:/dev$ traceroute www.superuser.com
   traceroute to www.superuser.com (151.101.129.69), 30 hops max, 60 byte packets    
   1  netiaspot.home (192.168.1.254)  2.904 ms  4.580 ms  6.440 ms
   2  83.238.252.80 (83.238.252.80)  28.329 ms  28.679 ms  30.332 ms
   3  83.238.113.24 (83.238.113.24)  41.348 ms  41.340 ms  41.310 ms
   4  83.238.252.62 (83.238.252.62)  42.953 ms  43.643 ms  43.633 ms
   5  83.238.248.183 (83.238.248.183)  47.110 ms  47.112 ms  48.809 ms
   6  149.6.28.97 (149.6.28.97)  54.555 ms  34.576 ms  38.036 ms
   7  130.117.3.29 (130.117.3.29)  44.751 ms  46.638 ms  48.214 ms
   8  154.54.38.205 (154.54.38.205)  55.980 ms  65.842 ms  69.702 ms
   9  * * *
   ...
   30  * * *
   
   
   sopel@mediacenter:/dev$ traceroute 192.168.1.1
   traceroute to 192.168.1.1 (192.168.1.1), 30 hops max, 60 byte packets
   1  * * *
   2  * * *
   3  * * *
   ...
   30  * * *
   

2. When only the modem is up

   sopel@mediacenter:/dev$ traceroute www.superuser.com
   traceroute to www.superuser.com (151.101.129.69), 30 hops max, 60 byte packets
    1  * * *
    2  * * *
    3  * * *
    4  * * *
    5  * * *
    6  185.89.184.2 (185.89.184.2)  104.232 ms  71.514 ms  82.738 ms
    7  * * *
    8  * * *
    9  * * *
   ...
   30  * * *
   
   sopel@mediacenter:/dev$ traceroute 192.168.1.1
   traceroute to 192.168.1.1 (192.168.1.1), 30 hops max, 60 byte packets
    1  * * *
    2  * * *
    3  * * *
    4  * * *
    5  * * *
    6  * * *
    7  * * *
    8  * * *
   ...
   30  * * *
   

Tenga en cuenta que en el segundo caso no se pudo acceder a 192.168.1.1 (no había ninguna LAN).