Firstly you do not have specifications like mathematical statistics in IISER's. You would have to choose a major in all the IISER's (except Pune I guess). If you take mathematics as your major and there are courses being offered in statistics (depending on various faculties in various IISER's such type of courses may or may not be present) you can take such courses and further your knowledge.

If you're even more interested you can do your MS thesis on such a topic.