my username starts with "ft"To be specific this -- FMT_tariff
LMFT, so if your username has "lfmt" in it, that would explain your NAT issues. Cause when LFMT is enabled, BSNL needs to give you a private IP address so that their LFMT service works properly.
Please confirm if this is the case. Otherwise, I don't see a reason for NAT issues.