Hi..
Isro conduct a exam for their hiring process and   to be a researcher/scientist in the organization you need bachelor's degree to apply for any position in Isro.
To become a math researcher to need to apply for scientist position which can only be applied if have the following degree.

• Engineering degree in cs/it/ec/me/ee
• BSc degree in maths/physics
• M.Sc(maths/physics) or mtech(maths)

You need these qualifications to apply for the required position. Once you cleared the written there will be an final interview.
Note : you can also apply in the final year.