As you are from a Mathematics background you would have numerous opportunities for your future.
Now you can go for research .
There are two primary fields for Mathematics
1.Pure Mathematics
2.Applied Mathematics.
1.Pure Mathematics
This a domain of mathematics where abstract theories are being explored .
Most of the students who actually choose mathematics for their love they Pursue their career in Pure Mathematics.
The Instructions which would be better are
1.ISI
2.CMI
3.IISER
4.IISc


2.Applied Mathematics
This is a domain where you apply your theories in real life.
This part of mathematics is in demand in Industry.
Artificial intelligence,Machine learning etc can be studied under this field.

The Instructions which would be better are
1.ISI
2.CMI
3.IISER
4.IISc

3. Miscellaneous
Now you can move to many departments after doing your higher studies in Maths.
Statistics is one of the best department where you can move as it has so much industrial application.
Data Science is another field where you can move to
Now Data Science is the future.
So most of the Maths guys choose this .
Hope this helps