If you really want to join ISRO in spite of not having mathematics in 12th is highly commendable decision.

You can join, but the process is bit longer. You will have to complete your graduation in B.sc Physics then you will have to complete M.sc and then you can apply for ISRO.

Let me remind you again that in these B.sc a=course and M.sc course there is lot of mathematics involved because bot these course are from Physics background.

So, you will have to work on your Mathematics too and that too on your own.

All the best!!