BSNL Nokia-Siemens H640V-G25E won't connect to the internet after trying to log in with an unregistered device.

Alright, then it should be fine. Looks like they have set the default VLAN to the BSNL internet VLAN.

For me, my LCO provides internet service from BSNL and another local ISP plus TV services etc through the same fiber infea. So the VLANs for each service differ and BSNL also isn't configured as the default. So have to specify the VLAN in my case.

Yes, it would vary with the location and the implementation.

I am curious to know whether the OP was able to connect internet with their new ONT.