Here is a shot in the dark, don't use VDSL but going by what I can see, you would need to change your default route the "DRoute" to your bridge interface which should be WAN4, since airtel uses VLAN tagging make sure it is tagged to whatever the pppoe2 is tagged to. You would also need to disable the modem's DHCP server for the LAN, unless it does that on its own in bridged mode.