Hello

ISRO recruits candidates from general science background too. But the eligibility for Scientist - SC is MSc in that stream. So you can join ISRO after your MSc in Maths upon vacancy. Recruitment happens for Scientist - SD for PhD candidates. ISRO has its own institute named IIST. If you are very much passionate about ISRO, then join IIST. AFAIK they take pupils for PhD. No idea about masters. Just go through their website.

I hope it will help you
Good luck...