See there are many entry points to ISRO. However the IIST B.Tech graduates have a chance to get absorbed in ISRO/DOS. To be eligible, candidates  must have completed a B.Tech at IIST with a minimum Cumulative Grade Point Average(CGPA) of 7.5 out of 10. However this is no guarantee that you will be absorbed. Check details here https://www.iist.ac.in/admissions/undergraduate. To go into IISt, you will have to clear JEE Advanced and get the requisite cutoff prescribed by the institute. For those details, pls check here https://engineering.careers360.com/articles/iist-btech-admission

There are various profiles one gets selected in ISRO. It depends on your qualification, your interests and branch specialization.