I have optlink modem from bsnl and have connected dlink router to it using lan and router configured itself automatically. Didn't do any setting for bridging.
I haven't changed any settings in bsnl modem.
Here is screenshot of Port forwarding settings Port Forward
Since not configured in the bridge, it's very likely that BSNL is the one dialling PPPoE and getting WAN IP. So before you setup port forwarding on your D Link router, you first need to setup port forwarding from BSNL Optilink device to D Link router.
Either setup port forwarding or just put D-Link device private LAN IP as "DMZ IP" in BSNL device and maintain firewall at the D-Link only.
Yes very likely because unless you have changed default setup:
Optilink will dial PPPoE and will get 1 Public WAN IP and next would have DHCP+NAT preconfigured. When you connect D-link to it and defined WAN IP for D-Link as "DHCP" then D-Link will learn a private IP via DHCP of Optilink.