@pufl You'll have to use static IP settings in
Windows on your desktop and put default gateway as 192.168.1.251 then you should be able to access your
ONT.
Once you are in ONT, you should enable NAT if its disabled, go to WAN section of ONT and set it to PPPoE and enter your creds. On BSNL you will also need to enter your VLAN I think, it could be configured already on the ONT by BSNL.