Change internet default route in Mac OS X to USB HSDPA modem stick with LAN’ certain subnet routed via a gateway in WiFi (en1).

In my case DNS will go with the modem interface setting remain routed via USB modem stick

before routing:

$ netstat -nr
 
Routing tables
 
Internet:
Destination        Gateway            Flags        Refs      Use   Netif Expire
default            10.64.64.64        UGSc           72      137    ppp0
default            link#4             UCSI            0        0     en0
default            10.1.1.1          UGScI           0        0     en1

Delete and add routes (desired LAN subnet 192.168.1.0/24:

sudo route -n delete default 10.1.1.1
sudo route -n add default 10.64.64.64
sudo route -n add 192.168.1.0/24 10.1.1.1

after routing:

$ netstat -nr
 
Routing tables
 
Internet:
Destination        Gateway            Flags        Refs      Use   Netif Expire
default            10.64.64.64        UGSc           21        0    ppp0
default            link#4             UCSI            0        0     en0
default            10.1.1.1          UGScI           0        0     en1
10.1.1/24         link#5             UCS             6        0     en1
...
192.168.1         10.1.1.1          UGSc            0        0     en1
...