3G GSM( ppp0) connected with LAN(eth0) connected not working

Below is the problem statement:-

1.3G GSM( Huewei E1320) connected.
2.LAN ethernet is connected.

State of the each one.

1. LAN(eth0) is connected to ASDL modem but without internet connected.
2. 3G GSM(ppp0) is connected is ready to take internet.

Now to the problem,
 This is a case where i have two internet connections, out of which one is temporarly not working.

        Actually i am trying to get internet connection from ppp0(3G GSM module) when eth0(LAN) is connected and without internet connection.

       So solve it, i tried routing to ppp0 as default. But not working when eth0 dont have any internet connection.

Say my ip route is as mentioned below:-

gateway 10.64.64.64 for ppp0
gateway 192.168.1.1 for eth0

I made the ppp0 as default, removing the eth0 from default route.
ip route add default via 10.64.64.64
ip route del default via 192.168.1.1
But internet not accessible.

   But when i plug the internet connection for LAN(eth0) by plugging internet cable to ADSL modem.Then i am able to get internet connection from ppp0(3G GSM module).

In short to summary:-

eth0 without internet connected and ppp0 with internet connection.
Default route is ppp0.
But still unable to access internet.

      I am out of steps to proceed in case of this problem. Can you confirm whether it is issue with network-manager.
I am specific stuck to this case.