@gcbeldar What does your LCO use? EPON/GPON?
Check the pinned thread on r/bsnl for ONT recommendations. Genexis Earth 1000R is decent, There is also another one from TP Link. Both are XPON units and should work regardless of whatever your LCO uses(epon/gpon).
Put the ONT in bridge mode and buy another decent router for routing + Wifi(or even better, Buy a dedicated router and then some APs to cover the area).