How to fix 'Your SIM cannot be registered' problem with Moto G2 on Marsh Mallow
I did some searching and it appears this is not restricted to yourself.  I mean if it works in another device (with full BSNL functionality) then you have limited the problem to this specific device.  I found the following page from a Moto G User which may still apply for you if you would like to give it a shot: 

http://www.techulator.com/resources/12632-Moto-G-not-recognizing-the-SIM-How-to-solve-SIM-problems.aspx

I am also wondering did you try to remove the SIM from the slot it is not working in plug it in to the slot it is not normally in and REBOOT the phone and see if you have it fully working.  If this is the case then it is probably an issue with Marshmallow recognizing the second slot which from looks of google search seems to be an issue they encountered when Lollipop upgrades first hit too.