BSNL FTTH configuration with TP Link dual band router

#1
Since I upgraded to BSNL'S 100mbps plan, I bought a dual band router. But I cannot seem to be able to configure it. My earlier ADSL router was configured in bridged mode to be used with ONT, but there doesn't seem to be any option for that in this router

Router model : TP Link Archer C20 AC750
 
#2
You don't need the router to be in bridge mode. You may just plug the LAN of ONT to wan port of your router.
In this case, you will end up with double NAT situation, ie your router and ONT doing NAT which in turn increases latency. To fix this you have many options such as configuring your ONT in bridge mode and dialing PPP via the router or Adding router's local IP to ONT's DMZ or run router as only in AP mode.