Hi,

Hope you are doing well.

You can join ISRO and DRDO as a mathematician, but a Bsc is insufficient; you must have an Msc or Phd. You can also enrol in universities such as IISC, IIST, and IIT to receive the greatest education possible. After completing your education, you can work at ISRO or DRDO as a scientist or assistant technician. Make sure you participate in internships or have some expertise.

Hope this helps.

Good Luck!