You need to set your ONT device in bridge mode, and then connect the cable from the ONT to the LAN port 4 as mentioned earlier by others. If you are unable to set the ONT to bridge mode no changes on D-Link router will work, cause it will simply do double NAT (not what you want).
@varkey i am on bsnl ftth with same routers as OP. And this particular ONT does not support bridge mode. but you can set dlink to bridge mode like i have explained in detail so that all the devices connected to dlink have their IP's assigned by ONT since dlink is on bridge mode. So there will be no double NAT issues since dlink is essentially doing nothing other than acting as a bridge between devices connected and the ONT.